Bioreactor for the cultivation of mammalian cells

ABSTRACT

The present invention relates to large-scale bioreactors having at least two impellers, large-scale bioreactor systems and methods for the large scale cultivation and propagation of mammalian cells using these bioreactors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser. No. 15/612,769, filed Jun. 2, 2017, which is a continuation application of U.S. application Ser. No. 14/885,802, filed Oct. 16, 2015, issued as U.S. Pat. No. 9,670,446 on Jun. 6, 2017, which is a divisional application of U.S. application Ser. No. 13/148,503, filed Aug. 23, 2011, now abandoned, which is a national stage application of International Application No. PCT/EP2010/000783, filed Feb. 9, 2010, which claims priority to European Application No. 09001755.9, filed Feb. 9, 2009, each of which is incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to bioreactors and methods for the large scale cultivation of mammalian cells using these bioreactors.

BACKGROUND OF THE INVENTION

It is important in mammalian cell culture processes to maintain the physicochemical environment in view of dissolved oxygen, culture pH, temperature and shear sensitivity. Also the maintenance of the nutritional environment is important. The maintenance of the cultivation conditions limits the possibility to perform large scale culturing of mammalian cells. Especially concentration gradients can inhibit the cell growth of mammalian cells in large-scale bioreactors.

One of the objects of the present invention is to provide bioreactors and methods, which allow the cultivation of mammalian cells in large scale volumes. Furthermore, it is an object of the present invention to provide bioreactors and methods, which allow the cultivation of mammalian cells under optimal conditions, even if grown in large scale volumes and therefore allow a process performance and product quality independent of the size of the bioreactor.

It is an object of the present invention to provide large-scale bioreactors which allow the cultivation of mammalian cells in a homogenous environment with respect to process parameters such as pH, dissolved oxygen tension (DOT) and temperature, maintaining a well mixed cell suspension and blending nutrient feeds within the bioreactor.

Furthermore it is an object of the present invention to provide devices and methods which allow the production of mammalian cells and products of the mammalian cells, especially proteins, peptides, antibiotics or amino acids, synthesised by the mammalian cells, in a large-scale manner.

DETAILED DESCRIPTION OF THE INVENTION

The present invention solves the technical problems underlying the present invention by the provision of bioreactors, bioreactor systems and methods for the cultivation of eukaryotic cells, especially of mammalian cells, according to the claims.

The present invention solves the technical problem underlying the present invention especially by the provision of a bioreactor for the cultivation of mammalian cells, characterised in that said bioreactor has at least two impellers. Furthermore, the present invention solves the technical problem underlying the present invention by the provision of a method for the cultivation and propagation of mammalian cells characterised in that at least one mammalian cell is cultivated under suitable conditions and in a suitable culture medium in a bioreactor, which has at least two impellers. Furthermore, the present invention solves the technical problems underlying the present invention by the provision of a bioreactor system for the cultivation of mammalian cells characterised in that a) a first bioreactor with a volume of at least 500 l is connected with b) a second bioreactor with a volume of at least 2000 l, which has a volume greater than the first bioreactor and wherein the second bioreactor with a volume of at least 2000 l is connected with c) a third bioreactor having at least two impellers and a volume of at least 10 000 l, which has a volume greater than the second bioreactor.

The present invention solves the technical problem underlying the present invention furthermore by the provision of a method to cultivate and propagate mammalian cells, characterised in that a) at least one mammalian cell is cultivated under suitable conditions and in a suitable culture medium in a first bioreactor with a volume of at least 500 l, b) the medium containing the cells obtained by propagation of the at least one mammalian cell is transferred into a second bioreactor with a volume of at least 2000 l, c) the transferred cells are cultivated in the second bioreactor with a volume of at least 2000 l, d) the medium containing the cells obtained in step c) is transferred into a third bioreactor with a volume of at least 10 000 l and e) the transferred cells are cultivated in the third bioreactor with a volume of at least 10 000 l.

According to the invention, the cultivated cells are eukaryotic cells, preferably animal cells, more preferably mammalian cells. The mammalian cells can be for example human cell lines, mouse myeloma (NS0)-cell lines, Chinese hamster ovary (CHO)-cell lines or hybridoma-cell lines. Preferably the mammalian cells are CHO-cell lines.

Preferably the cultivated cells are used to produce antibodies, more preferably monoclonal antibodies, and/or recombinant proteins, more preferably recombinant proteins for therapeutic use. Of course the cells may produce peptides, amino acids, fatty acids or other useful biochemical intermediates or metabolites. According to the invention the target concentration of the proteins produced by the cultivated cells is more than 0.5 g/l, preferably more than 2.0 g/l and most preferred more than 10.0 g/l. The method according to the invention can be used as a batch or in a fed-batch process. Although the cell-culture-medium used in the method according to the invention is preferably protein free medium, the design does not exclude the use of protein containing streams.

According to the invention a bioreactor is a biocompatible tank or vessel having additional equipment, for example impellers, baffles, spargers and/or ports, which specifically allows for the cultivation and propagation of mammalian cells. Preferably the tank or vessel is in the form of a tube, having on both ends of the tube, which build preferably the top and the bottom of the tank, plates. The plates are called head plate and base plate. In a particularly preferred embodiment of the present invention the base plate is an American Society of Mechanical Engineers Flanged and Dished (ASME F&D) designed base plate. The head-plate design preferably accommodates a manway or is preferably a flanged head plate to allow access/removal of the impellers.

The total tank height is the tangential line from the inner tank side of the base to the inner tank side of the head of the tank.

The freeboard height is defined as the length of straight side above the liquid head when the bioreactor is filled to it's operating volume. A minimum freeboard height is necessary taking into account the extent of foam build up during operation, gas hold up at maximum allowed agitation and aeration and errors in metering liquid.

The bioreactor according to the invention has a volume of preferably at least 500 l, more preferably of at least 1000 l, more preferably of at least 4000 l, even more preferably of at least 10 000 l, even more preferably of at least 20 000 l. Most preferably the bioreactor according to the invention has a volume of 1000 l, 1307 l, 4000 l, 5398 l, 20 000 l or 27 934 l.

Preferably, the bioreactor has a maximum volume of 100 000 l, more preferably the bioreactor has a maximum volume of 50 000 l, most preferably the bioreactor has a maximum volume of 30 000 l.

The design of the bioreactors according to the present invention ensures a homogenous environment with respect to process parameters such as pH, dissolved oxygen tension (DOT) and temperature, maintaining a well mixed cell suspension and blending nutrient feeds within the bioreactor. This provides the necessary physicochemical environment for optimal cell growth, product accumulation and product quality. The design of the bioreactors according to the present invention furthermore ensures the maintenance of geometric similarity. This allows a scale down model to be developed at 12 litre laboratory and 500 litre pilot scales.

The bioreactor for the cultivation of mammalian cells according to the invention has at least two impellers. More preferably, the bioreactor has two impellers, even more preferably a top impeller and a bottom impeller.

The bioreactor for the cultivation of mammalian cells according to the invention has preferably at least one top impeller and at least one bottom impeller, wherein the top impeller is preferably a hydrofoil impeller.

The bioreactor for the cultivation of mammalian cells according to the invention has preferably at least one top impeller and at least one bottom impeller, wherein the top impeller is a hydrofoil impeller.

The bioreactor for the cultivation of mammalian cells according to the invention has preferably a volume of at least 1000 l and at least one top impeller and at least one bottom impeller, wherein the top impeller is a hydrofoil impeller.

The bioreactor for the cultivation of mammalian cells according to the invention has preferably a volume of at least 4000 l and at least one top impeller and at least one bottom impeller, wherein the top impeller is preferably a hydrofoil impeller.

The bioreactor for the cultivation of mammalian cells according to the invention has preferably a volume of at least 4000 l and at least one top impeller and at least one bottom impeller, wherein the top impeller is a hydrofoil impeller.

In a preferred embodiment of the invention the top impeller is a hydrofoil impeller. The top impeller can be used preferably to provide strong bulk mixing.

In a preferred embodiment of the invention the bottom impeller is a hydrofoil impeller. In a preferred embodiment of the invention the top impeller and the bottom impeller are a hydrofoil impeller.

In a preferred embodiment of the invention at least the top impeller is a hydrofoil impeller. In a preferred embodiment of the invention all impellers are hydrofoil impellers.

According to a preferred embodiment of the invention the bottom impeller is a high-solidity pitch-blade impeller or a high-solidity hydrofoil impeller. The bottom impeller can be used preferably for the dissipation of sparged gas.

Preferably, the hydrofoil impellers provide much greater liquid motion, resulting in a greater bulk-mixing, for a given amount of power input. This can also depend of the flow number (N_(q)).

Preferably, non-hydrofoil impellers can provide liquid motion but at greater power inputs. This can have consequences on the health of shear-sensitive mammalian cells.

In a preferred embodiment of the invention the hydrofoil impeller is a down-flowing impeller or a up-flowing impeller.

In a preferred embodiment of the invention the top impeller is a down-flowing impeller. In a preferred embodiment of the invention the top impeller is a down-flowing axial hydrofoil impeller.

In a preferred embodiment of the invention the pulling down characteristics of the top impeller are used to mix the well aerated liquid surface with the liquid bulk.

In a preferred embodiment of the invention the hydrofoil impeller is a high efficiency hydrofoil impeller. In a preferred embodiment of the invention the hydrofoil impeller is a Chemineer—model SC-3 impeller, a LIGHTNIN—model A310 or A510 impeller, a Promix—model PHF series impeller or a Cleaveland Eastern Mixers impeller.

In a preferred embodiment of the invention the top impeller is a high efficiency hydrofoil impeller. In a preferred embodiment of the invention the top impeller is a Chemineer—model SC-3 impeller, a LIGHTNIN—model A310 or A510 impeller, a Promix—model PHF series impeller or a Cleaveland Eastern Mixers impeller.

The top impeller is preferably a three-bladed hydrofoil design impeller, for example a A310-type impeller from LIGHTNIN. The bottom impeller is preferably a four-pitched-bladed high-solidity impeller, for example of the A315-type from LIGHTNIN. The impeller to tank diameter ratio of the top impeller (D_(top)/T) and/or of the bottom impeller (D_(bottom)/T) is preferably at least 0.35 and at most 0.55, more preferably at least 0.40 and at most 0.48, and most preferably at least 0.44, and at most 0.46. A diameter greater than 0.5 results in disruption in axial flow, hence poor agitation and aeration.

The top impeller power number (N_(p)) is preferably at least 0.1 and at most 0.9, more preferably at least 0.25 and at most 0.35, most preferably 0.3. The top impeller flow number (N_(q)) is preferably at least 0.4 and at most 0.9, more preferably at least 0.50 and at most 0.60, most preferably 0.56. The bottom impeller power number (N_(p)) is preferably at least 0.5 and at most 0.9, more preferably at least 0.70 and at most 0.80, most preferably 0.75. The bottom impeller flow number (N_(q)) is preferably at least 0.50 and at most 0.85, more preferably at least 0.70 and at most 0.80, most preferably 0.73.

The impeller power number (N_(p)) is a measure of an impeller efficiency to impart the kinetic energy of the rotating impeller blades to the fluid. It is important in quantifying the gas dispersion. The impeller flow number (N_(q)) is a measure of pumping ability of the impeller and is important in quantifying fluid bulk movement.

The agitation rate of the at least two impellers is dependent on the scale. However, in a particularly preferred embodiment of the invention the agitation rate of the at least two impellers is at most 200 rounds per minute (rpm), more preferably at most 165 rpm.

The impeller spacing (D_(s)) is the space between the at least two impellers. It is in a particularly preferred embodiment of the invention at least 1×the diameter of the bottom impeller (D_(bottom)) and at most 2×D_(bottom), more preferably it is 1.229×D_(bottom) or 2×D_(bottom). This will allow both impellers to remain submerged at the lowest post-inoculation volume.

The liquid height above the upper impeller (D_(o)) is in a particularly preferred embodiment of the invention at least 0.3×the diameter of the top impeller (D_(top)) and at most 2.5×D_(top). More preferably it is at least 0.5×D_(top) and at most 2.0×D_(top).

The bottom clearance (D_(c)) is the clearance between the tank bottom and the centre-line of the bottom impeller. In a particularly preferred embodiment of the invention it is at least 0.35×D_(bottom), more preferably it is either 0.4×D_(bottom) or 0.75×D_(bottom).

The design of the impellers in the bioreactor according to the present invention provides optimal hydrodynamic characteristics in terms of bulk mixing, gas dispersion and low shear. The mammalian cells are kept in a homogeneous suspension by agitation via the impeller system according to the present invention.

The design of the impellers in the bioreactor according to the present invention provides rapid mixing, maintain homogeneity, maintain mammalian cells in suspension and gas bubble dispersion. The design of the impellers in the bioreactor according to the present invention minimises cell damage through shear forces originating from impeller geometry and eddies or vortices created behind the impeller blades.

In a particularly preferred embodiment of the present invention, the at least two impellers are a top driven agitator system.

The supply of air, especially compressed air, or specific gases, preferably oxygen, nitrogen and/or CO₂ is realised preferably through at least one sparger.

The bioreactor according to the invention has preferably at least one sparger, more preferably the bioreactor has one sparger or two spargers. The bioreactor according to the invention has preferably two spargers. Preferably the bioreactor has at least one sparger with a pipe-geometry. Preferably the at least one sparger is of the flute-type or is a sintered sparger. Preferably the at least one sparger is of the flute-type. In particularly preferred embodiment of the present invention a crescent pipe is explored. The curvature of the crescent is preferably 0.8×D_(bottom). In order to aid installation and removal from side ports of the bioreactor the crescent circumference is preferably 240° of the complete circumference of 0.8×D_(bottom) ring.

The at least one sparger provides sufficient oxygen mass transfer (characterised by K_(L)a) to meet the oxygen demand of the culture.

The at least one sparger provides a K_(L)a up to 20 h⁻¹ for cultures reaching up to 20×10⁶ cells per ml with an oxygen uptake rate of 5 mmol/I per hour. Two spargers used as a dual sparger system allow the removal of dissolved CO₂ and control of dissolved oxygen tension (DOT). Fluted spargers offer the benefits of easier cleaning in place (CIP) and sterilisation in place (SIP), aids with dCO₂ stripping and reduced operational costs as it is multiple use. Sintered spargers provide higher K_(L)a values. The lower intrinsic K_(L)a value with the fluted sparge design can be compensated by the use of oxygen enriched air. The gas flow rates are scaled up on the basis of constant superficial gas velocity.

It is important in large scale cultivation of mammalian cells to maintain a homogenous physicochemical environment in terms of dissolved oxygen, culture pH, and temperature, and dissolved CO₂, nutrient and metabolite concentration gradients. Whilst ensuring the physicochemical environment is homogenous through using appropriate agitation and aeration, it is important to ensure the selected operating agitation and aeration conditions do not produce adverse shear environment. The appropriate balance between ensuring homogenous environment that will promote good cell growth and productivity of mammalian cell culture processes whilst minimising the adverse effects of shear environment is dealt with in this invention. This is achieved through prescribing specific bioreactor geometries, impeller design and positioning, sparger design and positioning and specific operating limits for agitation and aeration rates.

The major damage to mammalian cells in stirred and sparged bioreactors comes from interfacial shear. Interfacial shear occurs as sparged gas bubbles coalesce and burst [ref: Ma N, Koelling K W, Chalmers J J. Biotechnol Bioeng. 2002 Nov. 20; 80(4):428-37. Erratum in: Biotechnol Bioeng. 2003 Feb. 5; 81(3):379]. Thus minimising sparged gas flows and excessive build up of foam is desirable. The interfacial shear can be minimised through a combination of approaches first by promoting surface aeration through good mixing of the liquid surface with the liquid bulk and secondly higher oxygen driving force by segregated oxygenation of cultures through the preferably two spargers.

The prescribed positioning of the hydrofoil impeller, particularly the liquid height above the upper impeller, D_(O) preferably being around 0.5×D_(top), below the liquid surface can aid strong and continuous exchange of the liquid surface with the liquid bulk thereby mixing the well oxygenated liquid surface with less oxygenated liquid bulk. The prescribed impeller spacing, preferably being D_(s)=1×D_(bottom) to 2×D_(bottom) can permit the down-flow of liquid generated by the upper impeller to feed fluid flow into the lower impeller thereby ensuring the whole fluid bulk is well-mixed and separate mixing zones are not made. The prescribed impeller bottom clearance, preferably being D_(c)=0.35×D to 0.75×D can ensure that the bulk flow is able to deflect off the curved ASME F&D base and rise upwards along the walls of the bioreactor.

The segregation of the ‘on-demand’ oxygenated sparged gas through the control sparger from the non-oxygenated sparged gases (such as CO₂, air and nitrogen ballasts) through a ballast sparger can allow greater residence time and path length of highly oxygenated sparge gas bubbles in the fluid bulk before disengaging out of the fluid bulk and into the headspace. This can permit greater oxygen transfer rates to be provided for a given volumetric mass transfer coefficient, k_(La). The residence time and path length of the sparged gas bubbles can be extended further through specifying down-flowing axial hydrofoil impellers that continuingly pull the liquid surface and liquid bulk down.

The bioreactor according to the invention has preferably at least one baffle, more preferably at least two baffles. The bioreactor according to the invention has most preferably four baffles.

Baffles are vertical radially located plates. Baffles are used to prevent the formation of a funnel or vortex formation.

In a preferred embodiment of the invention, the length of the at least one baffle is 1.1×the total straight height (H) of the bioreactor. The width of the baffle (W) is preferably 0.1×the internal diameter of the tank (T). The baffle clearance (W_(c)) is preferably 0.01×the internal diameter of the tank (T). The height of at least one baffle (H_(baffle)) is preferably 1.1×the total straight height (H)−the height of the bioreactor-head (H_(h)). Therefore H_(baffle) is preferably calculated according to the formula H_(baffle)=1.1×H−H_(h).

The thickness of the at least one baffle is not specified but the thickness needs to ensure rigidity to the radial component of the fluid flow. Additionally thickness needs to ensure the baffle plates are not warped during SIP thereby affecting the baffle to tank wall clearance.

The bioreactor according to the invention has preferably at least two ports for alkali addition. More preferably, the bioreactor has two ports for alkali addition. Most preferably, the bioreactor has two ports for alkali addition, wherein the first port is located at the central line of the bottom impeller and the second port is located at the central line of the top impeller. Preferably the pH probes are located diametrically opposite the alkali addition points into the bioreactor.

In a preferred embodiment of the invention, the bioreactor has a volume of 1000 l. The head volume (V_(h)) of a 1000 l bioreactor is preferably at least 45 l and at most 65 l, more preferably the head volume is 55 l. The base volume (V_(b)) of the 1000 l bioreactor is preferably at least 45 l and at most 65 l, more preferably the base volume is 55 l. The tank internal diameter (T) of the 1000 l bioreactor according to the invention is preferably at least 850 mm and at most 900 mm, more preferably the tank internal diameter is 864 mm. The tank cross-sectional area (A) of the 1000 l bioreactor according to the invention is preferably at least 0.55 m² and at most 0.65 m², more preferably the tank cross-sectional area is 0.586 m². The head height (H_(h)), which is the height of the head-plate, and/or the base height (H_(b)), which is the height of the base-plate, of the bioreactor with a volume of 1000 l according to the invention is preferably at least 120 mm and at most 180 mm, more preferably the head height and/or the base height is 151 mm. The total tank height of the 1000 l bioreactor according to the invention is preferably at least 2000 mm and at most 2600 mm, more preferably the total tank height is 2347 mm. The top impeller diameter (D_(top)) and/or the bottom impeller diameter (D_(bottom)) of the 1000 l bioreactor according to the invention is preferably at least 350 mm and at most 400 mm, more preferably the top impeller diameter and/or the bottom impeller diameter is 381 mm. The clearance between the tank bottom and centre-line of the bottom impeller (D_(c)) is for the 1000 l bioreactor according to the invention, preferably at least 120 mm and at most 180 mm, more preferably the clearance is 152 mm. The distance between the at least two impellers, also known as impeller separation (D_(s)) is for the 1000 l bioreactor, according to the invention, preferably at least 730 and at most 790 mm, more preferably the impeller separation is 762 mm. The impeller shaft diameter for the 1000 l bioreactor according to the invention is preferably at least 102 mm and at most 152 mm. If the 1000l bioreactor according to the invention has baffles, the length of the baffles is preferably at least 2000 mm and at most 2400 mm, more preferably the length is 2250 mm. The width of the baffles for the 1000 l bioreactor according to the invention is preferably at least 70 mm and at most 100 mm, more preferably the width is 86 mm. The baffle clearance for the 1000 l bioreactor according to the invention is preferably at least 7 mm and at most 11 mm, more preferably the baffle clearance is 9 mm. The baffle height (H_(baffle)) for the 1000 l bioreactor according to the invention is preferably at least 2000 mm and at most 2200 mm, more preferably the baffle height is 2099 mm. The 1000 l bioreactor according to the invention has preferably at least one sparger, more preferably it has one sparger. The at least one sparger of the 1000 l bioreactor according to the invention has preferably an orifice- or pore-size of at least 1.5 mm and at most 2.5 mm, more preferably the orifice- or pore-size is 2 mm. The orifice- or pore-number is preferably at least 20 and at most 40, more preferably the orifice- or pore-number is 30. The sparger length (S_(L) is preferably at least 150 mm and at most 550 mm, more preferably the sparger length is 305 mm. The sparger to tank bottom clearance (S_(c)) of the 1000 l bioreactor according to the invention is preferably at least 50 mm and at most 75 mm, more preferably the sparger to tank bottom clearance is 64 mm. The sparger to bottom impeller clearance (D_(c)−S_(c)) of the 1000 l bioreactor according to the invention is preferably at least 75 mm and at most 100 mm, more preferably the sparger to bottom impeller clearance is 88 mm.

In a preferred embodiment of the invention, the bioreactor has a volume of 4000 l. The head volume (V_(h)) of a 4000 l bioreactor is preferably at least 340 l and at most 370 l, more preferably the head volume is 359 l. The base volume (V_(b)) of the 4000 l bioreactor is preferably at least 340 l and at most 370 l, more preferably the base volume is 359 l. The tank internal diameter (T) of the 4000 l bioreactor according to the invention is preferably at least 1600 mm and at most 1650 mm, more preferably the tank internal diameter is 1626 mm. The tank cross-sectional area (A) of the 4000 l bioreactor according to the invention is preferably at least 1.90 m² and at most 2.30 m², more preferably the tank cross-sectional area is 2.076 m². The head height (H_(h)) and/or the base height (H_(b)) of the bioreactor with a volume of 4000 l according to the invention is preferably at least 260 mm and at most 300 mm, more preferably the head height and/or the base height is 282 mm. The total tank height of the 4000 l bioreactor according to the invention is preferably at least 2300 mm and at most 3100 mm, more preferably the total tank height is 2817 mm. The top impeller diameter (D_(top)) and/or the bottom impeller diameter (D_(bottom)) of the 4000 l bioreactor according to the invention is preferably at least 680 mm and at most 740 mm, more preferably the top impeller diameter and/or the bottom impeller diameter is 710 mm. The clearance between the tank bottom and centre line of the bottom impeller (D_(c)) is for the 4000 l bioreactor according to the invention, preferably at least 500 mm and at most 560 mm, more preferably the clearance is 531 mm. The distance between the at least two impellers, also known as impeller separation (D_(s)) is for the 4000 l bioreactor, according to the invention, preferably at least 840 mm and at most 900 mm, more preferably the impeller separation is 872 mm. The impeller shaft diameter for the 4000 l bioreactor according to the invention is preferably at least 51 mm and at most 64 mm. If the 4000 l bioreactor according to the invention has baffles, the length of the baffles is preferably at least 2200 mm and at most 2600 mm, more preferably the length is 2477 mm. The width of the baffles for the 4000 l bioreactor according to the invention is preferably at least 150 mm and at most 180 mm, more preferably the width is 163 mm. The baffle clearance is for the 4000 l bioreactor according to the invention preferably at least 12 mm and at most 20 mm, more preferably the baffle clearance is 16 mm. The baffle height (H_(baffle)) for the 4000l bioreactor according to the invention is preferably at least 2100 mm and at most 2300 mm, more preferably the baffle height is 2195 mm. The 4000 l bioreactor according to the invention has preferably at least one sparger, more preferably it has one sparger. The at least one sparger of the 4000 l bioreactor according to the invention has preferably an orifice- or pore-size of at least 1.5 mm and at most 2.5 mm, more preferably the orifice- or pore-size is 2 mm. The orifice- or pore-number for the 4000 l bioreactor according to the invention is preferably at least 80 and at most 120, more preferably the orifice- or pore-number is 100. The sparger length (S_(L)) is preferably at least 250 mm and at most 800 mm, more preferably the sparger length is 568 mm. The sparger to tank bottom clearance (S_(c)) of the 4000 l bioreactor according to the invention is preferably at least 315 mm and at most 360 mm, more preferably the sparger to tank bottom clearance is 337 mm. The sparger to bottom impeller clearance (D_(c)−S_(c)) of the 1000 l bioreactor according to the invention is preferably at least 180 mm and at most 205 mm, more preferably the sparger to bottom impeller clearance is 194 mm.

In a preferred embodiment of the invention, the bioreactor has a volume of 20 000 l. The head volume (V_(h)) of a 20 000 l bioreactor is preferably at least 1600 l and at most 2000 l, more preferably the head volume is 1803 l. The base volume (V_(b)) of the 20 000 l bioreactor is preferably at least 1600 l and at most 2000 l, more preferably the base volume is 1803 l. The tank internal diameter (T) of the 20 000 l bioreactor according to the invention is preferably at least 2500 mm and at most 3000 mm, more preferably the tank internal diameter is 2794 mm. The tank cross-sectional area (A) of the 20 000 l bioreactor according to the invention is preferably at least 5.8 m² and at most 6.5 m², more preferably the tank cross-sectional area is 6.131 m². The head height (H_(h)) and/or the base height (H_(b)) of the bioreactor with a volume of 20 000 l according to the invention is preferably at least 460 mm and at most 500 mm, more preferably the head height and/or the base height is 485 mm. The total tank height of the 20 000 l bioreactor according to the invention is preferably at least 4800 mm and at most 5100 mm, more preferably the total tank height is 4933 mm. The top impeller diameter (D_(top)) and/or the bottom impeller diameter (D_(bottom)) of the 20 000 l bioreactor according to the invention is preferably at least 1100 mm and at most 1300 mm, more preferably the top impeller diameter and/or the bottom impeller diameter is 1219 mm. The clearance between the tank bottom and centre line of the bottom impeller (D_(c)) is for the 20 000 l bioreactor according to the invention, preferably at least 890 mm and at most 945 mm, more preferably the clearance is 913 mm. The distance between the at least two impellers, also known as impeller separation (D_(s)) is for the 20 000 l bioreactor, according to the invention, preferably at least 1200 mm and at most 1700 mm, more preferably the impeller separation is 1498 mm. The impeller shaft diameter for the 20 000 l bioreactor according to the invention is preferably at least 51 mm and at most 64 mm. If the 20 000 l bioreactor according to the invention has baffles, the length of the baffles is preferably at least 4000 mm and at most 4600 mm, more preferably the length is 4365 mm. The width of the baffles for the 20 000 l bioreactor, according to the invention, is preferably at least 260 mm and at most 290 mm, more preferably the width is 279 mm. The baffle clearance for the 20 000 l bioreactor, according to the invention, is preferably at least 20 mm and at most 35 mm, more preferably the baffle clearance is 28 mm. The baffle height (Ha) for the 20 000 l bioreactor according to the invention is preferably at least 3600 mm and at most 4050 mm, more preferably the baffle height is 3882 mm. The 000 l bioreactor according to the invention has preferably at least one sparger, more preferably it has two spargers. If the 20 000 l bioreactor according to the invention has two spargers one is preferably a control sparger and one is preferably a ballast sparger. The control sparger for the 20 000 l bioreactor according to the invention has preferably an orifice- or pore-size of at least 3 mm and at most 5 mm, more preferably the orifice- or pore-size is 4 mm. The ballast sparger for the 20 000 l bioreactor according to the invention has preferably an orifice- or pore-size of at least 5 mm and at most 7 mm, more preferably the orifice- or pore-size is 6 mm. The orifice/pore number of the control sparger for the 20 000 l bioreactor according to the invention is preferably at least 230 and at most 270, more preferably the orifice- or pore-number is 250. The orifice-pore-number of the ballast sparger for the 20 000 l bioreactor according to the invention is preferably at least 85 and at most 115, more preferably the orifice- or pore-number is 100. The sparger length (S_(L)) for the control and/or the ballast sparger is preferably at least 500 mm and at most 2000 mm, more preferably the sparger length is 1077 mm. The sparger to tank bottom clearance (S_(c)) of the 20 000 l bioreactor according to the invention is preferably for the control and/or the ballast sparger at least 560 mm and at most 620 mm, more preferably the sparger to tank bottom clearance is 593 mm. The sparger to bottom impeller clearance (D_(c)−S_(c)) of the 20 000 l bioreactor according to the invention is for the control and/or the ballast sparger preferably at least 300 mm and at most 340 mm, more preferably the sparger to bottom impeller clearance is 320 mm. The requirement to add ballast from a separate sparger, the ballast sparger, prevents dilution of oxygen or oxygen enriched DOT demand gas with the ballast gas. This ensures the best oxygen transfer rate (OTR), as the oxygen concentration gradient of the bubbles emerging from the sparger is greatest. Secondly, the use of a ballast sparger allows spargers to be located at different positions to avoid impacting DOT control on delivering desired ballast for pCO₂ control. The ballast sparger can be independently designed from the control sparger.

With the bioreactor design according to the invention, different subculture ratios can be performed. In a particularly preferred embodiment the subculture ratios performed are subculture ratios of at least 1 in 5 (20% v/v) and at most 1 in 9 (11% v/v), more preferred 1 in 5 (20% v/v) or 1 in 9 (11% v/v).

The invention also includes a method to cultivate and propagate mammalian cells, characterised in that at least one mammalian cell is cultivated under suitable conditions and in a suitable culture medium in a bioreactor according the invention.

Bioreactors according to the invention include all bioreactors having at least two impellers and showing at least one feature or a combination of different features outlined above.

In the method according to the invention, the agitation rate of the at least two impellers of the bioreactor is preferably at least 55 W/m³ and at most 85 W/m³. Preferably, air is sparged into the culture medium with a speed of at least 5×10⁻⁵ m/s, more preferably of at least 10×10⁻⁵ m/s.

In a particularly preferred embodiment of the present invention alkali is added through two addition ports to distribute the alkali, which are, preferably spatially separated from each other. This ensures quicker blending of alkali in the event of long re-circulation time in the tank. CO₂ is preferably added via a control sparger.

Alkali and/or CO₂ are preferably used to regulate the pH of the culture-medium.

It is preferred that control and back-up probes be in the lower port ring at 913 mm from tank bottom.

In a preferred embodiment of the present invention, the method according to the invention takes place in a bioreactor with a volume of 1000 l. The volume of the culture medium used in the method using a 1000 l bioreactor is preferably during the pre-inoculation 50 l to 250 l. During the post-inoculation the volume of the culture medium is preferably at least 300 l and at most 960 l. In the pretransfer/harvest phase, the volume of the culture medium in the 1000 l bioreactor is preferably at least 300 l and at most 960 l. The minimum operating volume (V_(min)) in a bioreactor with the volume of 1000 l according to the invention is preferably between 80 l and 120 l, more preferably the minimum operating volume is 100 l, the maximum operating volume (V) is preferably at least 900 l and at most 1100 l, the maximum operating volume is more preferably 1000 l. The minimum stirred volume is preferably at least 230 l and at most 255 l, more preferably the minimum stirred volume is 245 l. The liquid height at the minimum operating volume (H_(min)) is in a bioreactor with a volume of 1000 l preferably at least 210 mm and at most 240 mm, more preferably the liquid height at the minimum operating volume is 228 mm. The liquid height at the maximum operating volume (H_(L)) in a bioreactor with a volume of 1000 l is preferably at least 1500 mm and at most 1900 mm, more preferably the liquid height at the maximum operating volume is 1764 mm. The minimum aspect ratio (H_(min)/T) is preferably at least 0.15 and at most 0.19, more preferably the minimum aspect ration is 0.17. The maximum aspect ratio (H_(L)−T) for the bioreactor with a volume of 1000 l used in a method according to the invention is preferably at least 1.8 and at most 2.1, more preferably the maximum aspect ratio is 1.96. The freeboard volume is preferably at least 270 l and at most 310 l, more preferably the freeboard volume is 293 l. The freeboard height is preferably at least 450 mm and at most 550 mm, more preferably the freeboard height is 500 mm. The total straight height (H) is preferably at least 1900 mm and at most 2200 mm, more preferably the total straight height is 2045 mm. The height of the upper probe- or sample-ring is preferably at least 900 mm and at most 1200 mm, more preferably the height of the upper probe- or sample-ring is 1093 mm. The height of the lower probe-sample ring is preferably at least 152 mm and at most 286 mm.

In a preferred embodiment of the present invention, the method according to the invention takes place in a bioreactor with a volume of 4000 l. The volume of the culture medium used in the method using a 4000 l bioreactor is preferably during the pre-inoculation 1914 l to 3077 l. During the post-inoculation the volume of the culture medium is preferably at least 2153 l and at most 3846 l. In the pretransfer/harvest phase, the volume of the culture medium in the 4000 l bioreactor is preferably at least 2153 l and at most 3846 l. The minimum operating volume (V_(min)) in a bioreactor with the volume of 4000 l according to the invention is preferably between 1500 l and 2200 l, more preferably the minimum operating volume is 1900 l, the maximum operating volume (V) is preferably at least 3800 l and at most 4200 l, the maximum operating volume is more preferably 4000 l. The minimum stirred volume is preferably at least 1500 l and at most 1800 l, more preferably the minimum stirred volume is 1654 l. The liquid height at the minimum operating volume (H_(min)) is in a bioreactor with a volume of 4000 l preferably at least 800 mm and at most 1200 mm, more preferably the liquid height at the minimum operating volume is 1024 mm. The liquid height at the maximum operating volume (H) in a bioreactor with a volume of 4000 l is preferably at least 1800 mm and at most 2200 mm, more preferably the liquid height at the maximum operating volume is 2034 mm. The minimum aspect ratio (H_(min)/T) is preferably at least 0.55 and at most 0.75, more preferably the minimum aspect ration is 0.63. The maximum aspect ratio (H_(L)−T) for the bioreactor with a volume of 4000 l used in a method according to the invention is preferably at least 1.1 and at most 1.4, more preferably the maximum aspect ratio is 1.25. The freeboard volume is preferably at least 850 l and at most 1250 l, more preferably the freeboard volume is 1039 l. The freeboard height is preferably at least 450 mm and at most 550 mm, more preferably the freeboard height is 500 mm. The total straight height (H) is preferably at least 2000 mm and at most 2400 mm, more preferably the total straight height is 2252 mm. The height of the upper probe- or sample-ring is preferably at least 1200 mm and at most 1600 mm, more preferably the height of the upper probe- or sample-ring is 1403 mm. The height of the lower probe- or sample-ring is preferably at least 500 mm and at most 550 mm, more preferably the height of the lower probe- or sample-ring is 531 mm.

In a preferred embodiment of the present invention, the method according to the invention takes place in a bioreactor with a volume of 20 000 l. The volume of the culture medium used in the method using a 20 000 l bioreactor is preferably during the pre-inoculation 13 913 l to 17 096 l. During the post-inoculation the volume of the culture medium is preferably at least 17 391 l and at most 19 231 l. In the pretransfer/harvest phase, the volume of the culture medium in the 20 000 l bioreactor is preferably at least 20 000 l and at most 21 l. The minimum operating volume (V_(min)) in a bioreactor with the volume of 20 000 l according to the invention is preferably between 9000 l and 16 000 l, more preferably the minimum operating volume is 13 000 l, the maximum operating volume (V) is preferably at least and at most 25 000 l, the maximum operating volume is more preferably 22 000 l. The minimum stirred volume is preferably at least 8100 l and at most 8500 l, more preferably the minimum stirred volume is 8379 l. The liquid height at the minimum operating volume (H_(min)) is in a bioreactor with a volume of 20 000 l preferably at least 2100 mm and at most 2500 mm, more preferably the liquid height at the minimum operating volume is 2309 mm. The liquid height at the maximum operating volume (H_(L)) in a bioreactor with a volume of 20 000 l is preferably at least 3550 mm and at most 3950 mm, more preferably the liquid height at the maximum operating volume is 3777 mm. The minimum aspect ratio (H_(min)/T) is preferably at least 0.70 and at most 0.99, more preferably the minimum aspect ration is 0.83. The maximum aspect ratio (H_(L)−T) for the bioreactor with a volume of 20 000 l used in a method according to the invention is preferably at least 1.2 and at most 1.5, more preferably the maximum aspect ratio is 1.35. The freeboard volume is preferably at least 5750 l and at most 6500 l, more preferably the freeboard volume is 6131 l. The freeboard height is preferably at least 900 mm and at most 1100 mm, more preferably the freeboard height is 1000 mm. The total straight height (H) is preferably at least 3700 mm and at most 4100 mm, more preferably the total straight height is 3968 mm. The height of the upper probe- or sample-ring is preferably at least 2200 mm and at most 2650 mm, more preferably the height of the upper probe- or sample-ring is 2411 mm. The height of the lower probe- or sample-ring is preferably at least 880 mm and at most 940 mm, more preferably the height of the lower probe- or sample-ring is 913 mm.

For a bioreactor with a volume of 20 000 l the preferred seeding ratio used is 11% v/v (1 in 9 dilution) or 20% v/v (1 in 5 dilution), with a preferred feed application of 4% v/v to 25% v/v of the post-inoculation volume. The post-inoculation volume in the 20 000 l bioreactor is preferably adjusted for feed applications up to 15% such that after the addition of all the feeds the final volume at harvest ends up at 20 000 l. However, for feed applications greater then 15% v/v the post-inoculation volume is preferably adjusted for a 15% v/v feed but following the application of feeds the final pre-harvest volume will be a minimum of 20 000 l and a maximum 22 000 l. The 20 000 l bioreactor is expected to hold a total of 20 000 l to 22 000 l at the end of a batch.

The bioreactor with a volume of 20 000 l is preferably operated in batch or fed batch mode for 10 to 15 days.

The invention also includes a bioreactor system for the cultivation of mammalian cells characterised in that a) a first bioreactor with a volume of at least 500 l, preferably of at least 1000 l, is connected with b) a second bioreactor with a volume of at least 2000 l, preferably of at least 4000 l, which has a volume greater than the first bioreactor and wherein the second bioreactor with a volume of at least 2000 l, preferably of at least 4000 l, is connected with c) a third bioreactor according to the invention having a volume of at least 10 000 l, preferably of at least 20 000 l, which has a volume greater than the second bioreactor.

In a preferred embodiment of the invention, the bioreactor system is characterised in that at least one of the bioreactors is a bioreactor according to the invention. More preferably, all of the bioreactors of the bioreactor system are bioreactors according to the invention.

Bioreactors according to the invention are in this context all bioreactors described in this description, in the examples and in the claims.

The bioreactor system according to the invention is also called bioreactor train or device.

The bioreactor train comprises preferably different bioreactors, which are also called stage. The bioreactor with a volume of at least 500 l, preferably of at least 1000 l corresponds to stage N-3 and/or N-2. The bioreactor with a volume of at least 2000 l, preferably of at least 4000 l corresponds to stage N-1. The bioreactor with a volume of at least 10 000 l, preferably of at least 20 000 l corresponds to stage N.

The design of the bioreactor train is based on the need to ensure a homogenous environment with respect to process parameters such as pH, dissolved oxygen tension (DOT) and temperature, maintaining a well mixed cell suspension and blending nutrient feeds within the bioreactor. The bioreactors of the bioreactor train preferably show geometric similarity. This allows a scale-down model to develop, for example at 12 l laboratory scales or 500 l pilot scales. The bioreactors of the stagCes N-3, N-2 and N-1 are used as seed-bioreactors. Bioreactor of stage N is used as a production-bioreactor. The design of the seed- and production-bioreactors is preferably based on the same principles. However, some departures can be required to allow for flexibility in processing.

In a preferred embodiment of the invention, the aspect ratio H_(L)/T is at least 0.17 and at most 1.96.

In a preferred embodiment of the invention there is a further bioreactor, especially a 50 l bioreactor corresponding to stage N-4.

In a preferred embodiment of the invention, the N-4 bioreactor is a S-200 seed wave bioreactor or a 100 l stirred tank reactor

In a preferred embodiment of the invention, liquids, for example culture medium, can be transported from one bioreactor to another bioreactor by pneumatic assisted flow or by peristaltic pumps.

The invention also includes a method to cultivate and propagate mammalian cells, characterised in that a) at least one mammalian cell is cultivated under suitable conditions and in a suitable culture medium in a first bioreactor with a volume of at least 500 l, preferably with a volume of at least 1000 l, b) the medium containing the cells obtained by propagation from the at least one mammalian cell is transferred into a second bioreactor with a volume of at least 2000 l, preferably with a volume of at least 4000 l, c) the transferred cells are cultivated in the second bioreactor with a volume of at least 2000 l, preferably with a volume of at least 4000 l, d) the medium containing the cells obtained in step c) is transferred into a third bioreactor with a volume of at least 10 000 l, preferably with a volume of at least 20 000 l, and e) the transferred cells are cultivated in the third bioreactor with a volume of at least 10 000 l, preferably with a volume of at least 20 000 l.

In a preferred embodiment of the invention, the method is characterised in that at least one of the bioreactors used is a bioreactor according to the invention, more preferably all bioreactors used are bioreactors according to the invention.

Bioreactors according to the invention are in this context all bioreactors described in this description, in the examples and in the claims.

The bioreactor of step e) is preferably operated in batch or fed batch mode. The cells are cultivated in step e) preferably for 10 to 15 days.

Step a) is also called stage N-3 and/or N-2. Step c) is also called stage N-1. Step e) is also called stage N.

Preferably the cultivation conditions in the bioreactors of steps a), c) and e) are the same. More preferably, the cultivation conditions in the bioreactors of steps a), c) and e) have a homogenous environment with respect to process parameters such as pH, dissolved oxygen tension and temperature. Preferably pH, dissolved oxygen tension and temperature in the bioreactors of steps a), c) and e) are the same.

In a preferred embodiment of the invention, the seeding ratio after the transfer steps b) and/or d) is at least 10% v/v, more preferably at least 11% v/v (1 in 9 dilution) and at most 30% v/v, more preferably 20% v/v (1 in 5 dilution).

Preferably either the total medium or only a part of the medium are transferred in steps b) and d).

Further preferred embodiments of the present invention are the subject-matter of the sub claims.

The present invention is illustrated in more detail in the following examples and the accompanying figures.

FIG. 1 shows a bioreactor according to the invention. 1 is the bioreactor. 10 is the diameter of the tank (T). 20 is the total straight height of the bioreactor (H). 30 is the base height of the bioreactor (H_(b)). 40 is the head height of the bioreactor (H_(h)). 50 is the liquid height at the maximum operating volume (H_(L)). 60 is the top impeller diameter (D_(top)). 68 is the top impeller. 70 is the bottom impeller diameter (D_(bottom)). 78 is the bottom impeller. 80 is the clearance between tank bottom and centre line of the bottom impeller (D_(c)). 90 is the impeller separation (D_(s)). 100 is the clearance of the top impeller below the liquid surface (D_(o)). 108 is a sparger. 110 is the sparger to tank bottom clearance (S_(c)). 120 is the sparger to bottom impeller clearance (D_(c)−S_(c)). 128 is a baffle. 138 is a port located at the lower ring. 148 is a port located at the centre-line of the top impeller 68.

FIG. 2 shows a bioreactor system of the present invention. 111 is a bioreactor with a volume of 1000 l. 11 is a bioreactor with a volume of 4000 l. 1 is a bioreactor according to the invention with a volume of 20 000 l.

EXAMPLE 1: 20 000 L BIOREACTOR

The 20 000 l bioreactor is operated in batch and fed batch mode for to 15 days for the cultivation of mammalian cells. The mammalian cells are kept in a homogeneous suspension by agitation via an impeller system.

Vessel Geometry

The vessel geometry for the 20 000 litre bioreactor was determined by an iterative design basis in which the maximum working volume, freeboard straight side distance, aspect ratio H_(L)/T and impeller to tank diameter, D/T ratio are altered until an acceptable aspect ratio is achieved.

Bioreactor Aspect Ratio H_(L)/T

This critical design parameter allows characterisation of bioreactor geometry. Tanks with higher aspect ratio offer longer gas residence time allowing greater K_(L)a. However increased head pressure can cause build up of soluble gases. Smaller aspect ratio H_(L)/T in tanks can lead to shorter gas residence time requiring greater gas flow for aeration resulting in greater foam build up. Impeller driven agitation to increase K_(L)a is also limited by H_(L)/T as surface breakage and vortex creation will occur at lower impeller revolutions in a low aspect ratio. Thus choice of aspect ratio is largely experience based with some thought on issues highlighted in table 1.

TABLE 1 Summary of effect of varying aspect ratio Process factor High aspect ratio Low aspect ratio Radial mixing More effective Less effective Mixing time Higher Lower Oxygen transfer rate Determined by Determined by dissolved oxygen dissolved oxygen control control Gas flow rate Lower Higher Cell damage Less More Carbon dioxide stripping Less effective More effective Pressure variations Higher Lower Ease of scale More difficult away More difficult away up/scale down from currently used from currently used (access to scale data) aspect ratios aspect ratios Cleanability Not affected directly Not affected directly by aspect ratio by aspect ratio Volume flexibility Less More

Table 2 describes the aspect ratios in the 20 000 litre bioreactor at various operating volumes during normal processing. The aspect ratios have been tested at 500 litre scale and provided the superficial gas velocity and power per unit volume are kept constant the K_(L)a remains constant.

TABLE 2 Key operating volumes and aspect ratios in the 20000 liter bioreactor Volume, Liquid head, Aspect ratio, L mm H_(L)/T Pre-Inoculation 13913-17096 2458-2977 0.88-1.07 Post Inoculation 17391-19231 3025-3325 1.08-1.19 Harvest 20000-21739 3451-3734 1.23-1.34 Tank Diameter

The tank diameter is altered to obtain the optimal aspect ratio H_(L)/T. Changes to tank internal diameter (ID) are limited by acceptable aspect ratio and plant footprint. The ID is 2.794 m.

Tank Height

Tank height is determined from the maximum operating volume, aspect ratio H_(L)/T, freeboard straight side length, base and top plate design. The final tank height is a compromise value determined from volumetric contingency for foam, plant height and impeller shaft length. The tank height from base to head tan line is 4.933 m.

Freeboard Height

The freeboard height is defined as the length of straight side above the liquid head when the bioreactor is filled to it's maximum operating volume. This is determined by taking into account the extent of:

-   -   Foam build up during operation.     -   Gas hold up at maximum allowed agitation and aeration.     -   Errors in metering liquid.

In absence of knowing the exact contribution of each with piloting the process at full scale an estimate is usually made. The amount of freeboard height is balanced with the desire to reduce the impeller shaft length for a top-driven system, where extra length can complicate the design and selection of available mechanical seals, the requirement for steady bearing or stabilising impeller rings. A minimum freeboard height of 1000 mm (or 6100 litre volumetric capacity or 28% v/v of the maximum operating volume) is therefore used.

Head and Base Plate

The selection of head and base plate design was made with a consideration for desired mechanical strength, free draining clean design and fluid flow. Maintaining consistent plate design between scale down and full scale will contribute towards maintaining geometric similarity. The base plate is of American Society of Mechanical Engineers Flanged and Dished (ASME F&D) design. The head-plate design accommodates a manway or a flanged head plate to allow access/removal of the impellers.

Bioreactor Agitation Requirement

The agitation of the bioreactor is to achieve rapid mixing, maintain homogeneity, maintain mammalian cells in suspension and gas bubble dispersion. The underlying issue with achieving the above objectives is minimising cell damage through shear forces originating from impeller geometry and eddies or vortices created behind the impeller blades. A compromise of the above objectives can be achieved by selection of an appropriate impeller type.

Bottom Versus Top Driven Impeller Shaft

The decision to drive the agitator shaft from the top or the bottom of the bioreactor is important and is determined following a review of a number of issues highlighted in table 3.

TABLE 3 Key design issues for selection of top versus bottom entry of impeller shaft Top entry Bottom entry Shaft Length Long Short Shaft Weight High Low Shaft Diameter Larger Smaller Impeller shaft on-site Greater plant height Less plant height installation and removal for servicing and repair Exposure of cell culture to No exposure Exposure moving and stationary seal faces ¹Pressurization between Lower Higher due to seal and vessel the liquid head Seal Lubricant leakage rate Lower Higher Base plate Design Simple Complex Sparger to tank bottom Unrestricted Restricted positioning CIP validation Simple Complicated by submerged mechanical seal Scale up and scale down Consistent Inconsistent consistency with lab and with lab and pilot scale pilot scale ¹Pressure differential between seal and bioreactor critical for lubrication and cooling.

Top-entry impeller shafts tend to be longer than bottom-entry, which results in the shaft being heavier and larger diameter. Additionally the shaft length together with the inherent clearance between the two faces of the mechanical seal may dictate the requirement for steady bearings or stabilising ring to prevent excessive “shaft wobble”. Service and maintenance are affected by the available space around the agitator, gearbox and seal assembly, and on-site shaft installation and removal is limited by plant height.

The protrusion of the seal and impeller shaft at tank bottom restricts the placement of the sparger near the tank bottom. This dimension affects the tank hydrodynamics and therefore its amenability to change is important in specifying an optimal design.

The downwards load of down pumping impellers together with the liquid head have an accumulative greater load (compared to up pumping or top-entry shaft) between the moving and stationary faces of the seal resulting in greater wear of the seal faces. Furthermore loss of over pressure in the condensate line supplying the seal can result in the culture seeping into the seal. This makes the subsurface seal a less sanitary design.

The submerged seal complicates the design of a free draining bioreactor by compromising the position of the harvest drain valve. Secondly the diameter of the harvest nozzle may be restricted thus restricting the flow rate of harvest stream. Therefore a top entry impeller shaft is used in the 20 000 litre bioreactor.

Baffles

The baffle requirement for centre mounted impeller is critical to prevent vortex formation. The critical issues related to baffles are baffle number, baffle width (W), baffle length (H_(baffle)) and baffle to tank wall clearance (W_(c)).

The recommendation for four equally spaced baffles that are 0.1×T or 279 mm wide 1.1×H−H_(h) or 3882 mm tall and have a baffle to tank wall clearance, W_(c) of 0.01×T or 28 mm.

The thickness of baffle is not specified but the thickness needs to ensure rigidity to the radial component of the fluid flow. Additionally thickness needs to ensure the baffle plates are not warped during SIP thereby affecting the baffle to tank wall clearance.

Impeller Type

High shear, such as Rushton (or Rushton-type), impellers offer high power dissipation for gas dispersion but lack in axial flow necessary for mixing and homogeneity. Additionally, agitation from high shear impellers suffers from dangers of excessive cell damage.

Table 4 shows the impellers tested at lab scale (12.2 litre) that gave equivalent hydrodynamic and cell growth performance. The hydrofoil is mounted above the high solidity pitched blade impeller.

The Lightnin A310 and A315 at the D/T ratio described in table 4 are used in the bioreactor.

TABLE 4 Impeller types short-listed for scale down study Impellers D/T ratio ¹N_(p)/²N_(q) Vendor Description A310 0.44 0.30/0.56 Lightnin Three bladed hydrofoil design A315 0.46 0.75/0.73 Lightnin Four pitched- bladed high solidity impeller SC-3 0.40 0.90/0.90 Chemineer Three bladed hydrofoil design 3HS39 0.46 0.53/0.58 Philadelphia Four pitched- Mixers bladed high solidity impeller ¹N_(p) is characteristic impeller power number. It is a measure of an impeller efficiency to impart the kinetic energy of the rotating impeller blades to the fluid. It is important in quantifying the gas dispersion ²N_(q) is characteristic impeller flow number. It is a measure of pumping ability of the impeller and is important in quantifying fluid bulk movement. Impeller to Tank Diameter, D/T Ratio

The diameter for axial flow impellers is recommended to be less than 0.5×T. A diameter greater than this results in disruption in axial flow, hence poor agitation and aeration.

Power dissipation into the bioreactor and Reynold's number also need to be sufficiently high to maintain a turbulent (loaded) regime. Therefore the selection of impeller diameter is a compromise between choosing large enough diameter to ensure adequate homogeneous mixing without exceeding the hydrodynamic characteristics of the bioreactor. These include throttling axial flow, insufficient power dissipation, exceeding upper limits of impeller tip speed and creation of poorly mixed laminar zone.

Once a diameter is selected, than maintaining constant D/T ratio is critical between scale down pilot vessels in order to maintain the central assumption of scale studies—that of maintaining geometric similarity.

The K_(L)a scale up correlation at 12.2 litre has been determined for the four impellers at the D/T ratios shown in table 4. From a geometric similarity standpoint A310 diameter of 1.229 m (D/T of 0.44) and A315 diameter of 1.285 m (D/T of 0.46) is recommended. However a manway diameter can restrict the largest impeller diameter that can be installed and removed to 1.219 m. Therefore A310 and A315 to be 1.219 m diameter are used thereby keeping with ease of impeller installation and removal and maintaining close to the geometric similarity proposed in scale down study.

The Impeller Clearance, D_(c) and Spacing, D_(s)

The spacing between impellers in a bioreactor with multiple impellers is an important dimension to consider. For a bioreactor with dual Rushton turbine (radial flow) the ungassed power consumption is equivalent to a single impeller when the dual impeller are spaced less then 0.5×D along the shaft. At a spacing of 2×D the power consumption becomes adductive. Thus efficiency of the impeller is reduced when the impeller spacing becomes less then 0.5×D and the requirement for multiple impellers becomes unnecessary. It is important to note that impeller spacing also impacts on the potential of creating dead zones (poorly mixed zones) within the bioreactor. An additional constraint on the choice of impeller spacing is discrete working volumes required within the bioreactor.

The impeller spacing, D_(s), of 1.229×D_(bottom) (1498 mm) allows both impellers to remain submerged at the lowest post-inoculation volume of 17392 litres with liquid head above the upper impeller, D_(o), of 0.5×D_(top) (615 mm) and off bottom clearance, D_(c), of 0.75×D_(bottom) (913 mm).

Table 5 highlights volumes that will form liquid surfaces or lower liquid cover, above the impellers. Agitation needs to be modified to avoid foaming at these critical volumes.

TABLE 5 Key operating volumes that cause interaction with impellers and liquid surface Interaction Volume, L Potential Operation Submerge top impeller with 17399 Minimum post inoculation 0.5D_(A310) liquid cover volume 17391 L Liquid surface touching top 13973 Pre-inoculation volume of edge of top impeller 13913 L liquid surface breakage Liquid surface touching 13283 Bolus addition of pre- bottom edge of top impeller inoculation medium will pass through this liquid head Submerge bottom impeller  8381 Bolus addition of pre- with 0.5D_(A315) liquid cover inoculation medium will pass through this liquid head Liquid surface touching top  5592 Bolus addition of pre- edge of bottom impeller inoculation medium will pass through this liquid head Liquid surface touching  3291 Bolus addition of pre- bottom edge of bottom inoculation medium will pass impeller through this liquid head ⁽¹⁾ Minimum operating volume with lower impeller submerged is 8379 liters and minimum operating volume with both impellers submerged is 17399 liters ⁽²⁾ The operating volume range is 13913 to 21739 liters. Clearance of Top Impeller Below Liquid Surface, Do.

The breakage of the impeller blade above liquid surface is undesirable as this will make the flow and power dissipation of the impeller ineffective. In addition it will create unknown K_(L)a values due to significant surface entrapment of headspace gas into the fluid and excessive foam. D_(o) is 0.3×D for radial flow impellers and 0.5×D for axial flow impellers such as A310. However as D_(o) approaches 2×D the impeller provides gentle blending duty. This is acceptable for the production bioreactor application as K_(L)a study has shown that bioreactor K_(L)a is influenced mostly by the bottom A315 impeller and the top A310 impeller contributes to bulk mixing.

As a result of setting D_(c) and D_(s) at values D_(o) is maintained at an optimal range for the duration of operation of the production bioreactor. During the course of a batch the liquid cover above the top impeller will change from 0.5×D_(A310) and 1.08×D_(A310). The liquid cover above the top impeller will increase as the bioreactor is fed nutrient feeds and alkali to maintain constant pH. Table 6 shows a range of liquid cover above the top impeller for a range of operating volumes.

TABLE 6 Key operating volumes and the liquid cover above top impeller, Do Cylinderical Do as height, H mm Do, mm ratio of Operating volume, L (inches) (inches) D_(A310) Pre-Harvest, 21739 L 3252 (128″) 1324 (52″) 1.08D_(A310) Pre-Harvest, 20000 L 2968 (117″) 1040 (41″) 0.85D_(A310) Post-Inoculation, 19231 L 2843 (112″) 915 (36″) 0.74D_(A310) Post-Inoculation, 17391 L 2543 (100″) 615 (24″) 0.5D_(A310) Pre-Inoculation, 15385 L 2215 (87″) 287 (11″) 0.23D_(A310) Pre-Inoculation, 13913 L 1973 (78″) 45 (2″) 0.04D_(A310) ⁽¹⁾ Off bottom impeller clearance, Dc = 913 mm (0.75D_(A315)), Impeller separation, Ds = 1498 mm (1.229D_(A315)), tank ID of 2794 mm and height of ASME F&D base plate, H_(h) = 483 mm ⁽²⁾ D_(o) = H − D_(s) − (D_(c) − H_(h)) Agitation Rate—Rpm, P/V and Tip Speed

Table 7 below specifies the agitation rate for the 20 000 litre bioreactor. The bioreactor is agitated typically at 20-260 W/m³, preferably at 55-85 100 W/m³. The agitation strategy is being developed during the 500 litre pilot fermentations. The agitation rate of 0 to 80±1 rpm is therefore used as an operational range.

TABLE 7 Agitation rate for the 20000 L bioreactor Agitation rate, Power per unit volume, Tip Speed, rpm W · m⁻³ m/s Pre- Typically 28-30 can Typically 20 can 1.8-1.9 inoculation be higher be higher Post- Typically 56 can be Typically 103, can be 3.6 can be inoculation up to 80 up to 260 up to 5.1 until harvest Mechanical Seals Specification

For bioreactor all seals are to be double mechanical seals with a maximum “run out” or wobble tolerance of 0.2 mm. Three types were considered; these include:

-   -   Wet seal lubricated with sterile condensate.     -   Dry seal lubricated with sterile gas such as N₂ or CA.     -   Non lubricated or floating seal that are uni-rotational.

All mechanical seals are recommended to be serviced on an annual basis. This requires the removal of seal from the bioreactor and sending the seal assembly to the vendor. Therefore the design must consider ease of routine maintenance.

The dry type seal (John Crane—5280D type) will produce 3 g per year of shedding (seal face and seal seat material) composed of resin impregnated carbon. This is based on continuous 24 hour operation over a year. The amount of shedding for the wet seal is significantly less. Therefore a wet condensate-lubricated seal is adopted for all bioreactor double seals.

Bioreactor Aeration Requirement and Gassing Strategy

The aeration duty of the 20 000 litre bioreactor is governed by:

-   -   K_(L)a requirement.     -   DOT control strategy.     -   pCO₂ control/stripping strategy.     -   Use of sintered or fluted spargers.

The 20 000 litre bioreactor is designed to provide K_(L)a values of up to 20 h⁻¹ for processes with oxygen uptake rates of 5 mmol×L⁻¹×h⁻¹. The bioreactor design needs to be flexible enough to allow cultivation of processes reaching 20×10⁶ cells×mL⁻¹.

The aeration requirement can be achieved by a number of different approaches. However the use of a fluted sparger with air and oxygen enrichment to make up any deficit in oxygen transfer rate (OTR) during peak oxygen demand was used. The advantages of this approach are:

-   -   Easier CIP and SIP validation of fluted sparge design.     -   Larger air throughput to aid dissolved CO₂ stripping.     -   Reduced operating cost through the avoidance of purchase of         single use sintered elements.

The disadvantages of the approach selected above also need to be considered. These include:

-   -   Inherent lower K_(L)a for the low power number impellers         selected.

Therefore the bioreactor aeration design must have the flexibility to be modified to meet the desired K_(L)a.

Table 8 describes the gassing requirements for the 20 000 litre bioreactor. The gas flow rates were scaled up on constant superficial gas velocity.

Two spargers are used. The main or “DOT control” sparger supplied by dual range clean air, mass flow controller (MFC) and oxygen MFC with gas flow metered via a DOT control loop and a CO₂ MFC metering gas via the acid pH control loop. The dual range MFC's are used to achieve precise flow control at the extreme ends of the desired operating ranges.

The second or “ballast” sparger is supplied by a CA MFC to which nitrogen is also supplied. It was measured that early DOT control requires small nitrogen ballast to assist in early DOT demand and lower the DOT to set point. The ballast sparger also meters ballast air to facilitate stripping out excess pCO₂.

The headspace purge is used to allow removal of CO₂ and oxygen from the headspace. This is to facilitate better pH and pCO₂ control and dilution of high oxygen blend prior to exhausting to environment. The ability to vary headspace flow rate allows design of gassing strategy for various processes requiring different blends of oxygen enrichment and control point pCO₂.

TABLE 8 Gas flow rate and MFC operating ranges for the 20000 liter bioreactor Gas Operating range Comments Head Space¹ 1.) Clean air 1.) 0-1000 SLPM 1.) Head space purging of CO₂ and O₂ 2.) Nitrogen 2.) Utility rated 2.) For rapid DOT probe zeroing 3.) Helium 3.) Utility rated 3.) Tank integrity testing DOT control Sparger 1.) Clean air² 1.) 10-500 SLPM 1.) Gas flow under DOT control 2.) Oxygen 2.) 10-100 SLPM 2.) Gas flow under DOT control Carbon dioxide³ 3.) 2-150 SLPM 3.) Gas flow under pH control Ballast Sparger 1.) Clean air 1.) 20-500 SLPM 1.) Variable ballast for dCO₂ stripping 2.) Nitrogen⁴ 2.) 20-500 SLPM 2.) Early DOT control by variable flow ¹The air and nitrogen gas flow into headspace enters via a bypass for post SIP tank pressurisation. ²Clean air gas flow operating range achieved by a dual CA MFC at 5-50 SLPM and 50-500 SLPM respectively. ³CO₂ gas flow operating range achieved by a dual CO₂ MFC at 2-30 SLPM and 30-150 SLPM respectively. ⁴Both air and nitrogen gas flow metered from a common CA MFC.

The bioreactor ports for sparger installation are designed to fit pipe design of diameter of 51 mm. The position of port should allow the placement of control sparger (D_(c)−S_(c)) at a distance of 320 mm below the bottom edge of the lower impeller and no greater 593 mm from tank bottom (S_(c)).

This results in a S_(c) value of 593 mm or (0.65×D_(c)) and this falls outside the acceptable range of 0.2×Dc to 0.6×Dc. However hydrodynamic trials in 500 l suggest S_(c) clearance of 0.41 to 0.71×D_(o) has no impact on measured K_(L)a.

A separate port for the installation of the ballast sparger was also built. The position of this port allows the placement of ballast sparger at a distance of 320 mm, (D_(c)−S_(c)) below the bottom edge of the lower impeller and no greater then 593 mm from tank bottom (S_(c)). The requirement to add ballast from a separate sparger is due to three reasons:

-   -   Firstly, it prevents dilution of oxygen or oxygen enriched DOT         demand gas with the ballast gas. This ensures the best OTR, as         the oxygen concentration gradient of the bubbles emerging from         the sparger is greatest.     -   Secondly, it allows ballast sparger to be located at a different         position from DOT control sparger to avoid impacting DOT control         on delivering desired ballast for pCO₂ control.     -   Thirdly, the ballast sparger can be independently designed from         the DOT control sparger.

The calculation of hole size and number of holes is iterated until the target Reynold's number, Re of gas emerging from holes is <2000 and the Sauter mean diameter for a bubble is 10-20 mm during chain bubble regime. Table 9 shows the key specifications for the control and ballast sparger for the 20 000 litre bioreactor.

TABLE 9 Design specification for the 20 000 liter bioreactor spargers DOT control Ballast Parameter sparger sparger Gas flow, SLPM 850 500 Number of sparge holes 250 100 Orifice diameter, d_(o), m 0.004 0.006 Gas flow, m³ · s⁻¹ 1.42E−02 8.33E−03 Orifice area, m² 1.26E−05 2.83E−05 Total orifice area, m² 3.14E−03 2.83E−03 Density of air, Kg · m⁻³ 1.166 1.166 Viscosity, Nm · s⁻² 1.85E−05 1.85E−05 Sauter mean diameter, d_(vs), mm (d_(vs) = 16.34 19.06 1.17 V_(o) ^(0.4)d_(o) ^(0.8)g^(−0.2)) Gravitational acceleration, g m · s⁻² 9.807 9.807 Density difference, Kg · m⁻³ 1048.834 1048.834 Reynold's number, >2000 jetting regime 1139 1117 Gas velocity at sparger, V_(o), m/s 4.51 2.95 Sparger length, S_(L), m 1.077 1.077 Combined length to drill required holes, m 1.000 0.6 Number of rows to fit required holes in length S_(L) 2 1 Sparger to tank bottom clearance, Sc, m 0.593 0.593 Sparger to bottom impeller clearance, Dc-Sc, m 0.320 0.320

A ring sparger of 0.8×D_(bottom) (80% diameter of bottom A-315 impeller diameter) is used to distribute the holes under the blades and not the impeller hub. However the CIP and installation of this configuration is difficult. Therefore selection of sparger geometry that permits distribution of the desired number of holes in a manner that is consistent with best to distribute the holes and sanitary design can be used also.

As an option a crescent rather then straight pipe is explored. The curvature of the crescent is 0.8×D_(bottom). In order to aid installation and removal from side ports of the bioreactor the crescent circumference is 240° of the complete circumference of 0.8×D_(bottom) ring, this is 1077 mm.

The DOT control sparger is 1077 mm long and has a 51 mm diameter. The holes have a 4 mm diameter. A total of 250 holes divided into 2 rows (2×125) at 45° from the dorsal (vertical) are used. Drain holes of 4 mm diameter on both ends of the sparger are drilled on the ventral side of the sparger to aid free CIP drainage of the sparger.

The ballast sparger is 1077 mm long and of 51 mm diameter and has a total of 100 6 mm diameter holes in a single dorsal row. Drain holes of 4 mm diameter on both ends of the sparger are drilled on the ventral side of the sparger to aid free CIP drainage of the sparger.

Position of Probes, Addition and Sample Ports

The probe ring position must be placed in a well-mixed representative region of the bioreactor. Additional considerations included working volume range and ergonomic operations. The location of probe ports, sample valve and addition points were considered together to avoid transitory spikes. Furthermore the position of the sample valve with respect to controlling probes needs to permit accurate estimation of off-line verification of the measured process parameter. This is shown in table 10.

TABLE 10 Probe, addition and sampling port specification for the 20000 liter bioreactor ²Diameter, ¹Position, Probe/Port Location mm (inches) mm (inches) Rational Temperature Lower 38.1 (1.5″) 1.) 913 (36″) In the plane of (main) ring 2.) 30° centre-line of bottom impeller Temperature Lower 38.1 (1.5″) 1.) 913 (36″) In the plane of (backup) ring 2.) 170° centre-line of bottom impeller pH Lower 38.1 (1.5″) 1.) 913 (36″) In the plane of (main) ring 2.) 10° centre-line of bottom impeller pH Lower 38.1 (1.5″) 1.) 913 (36″) In the plane of (backup) ring 2.) 20° centre-line of bottom impeller DOT Lower 25.0 (0.98″) 1.) 913 (36″) In the plane of (main) ring 2.) 150° centre-line of bottom impeller DOT Lower 25.0 (0.98″) 1.) 913 (36″) In the plane of (backup) ring 2.) 160° centre-line of bottom impeller pCO₂ Lower 50.8 (2″)tbd 1.) 913 (36″) In the plane of (spare) ring 2.) 20° centre-line of bottom impeller Biomass Lower 50.8 (2″) 1.) 913 (36″) In the plane of (spare) ring 2.) 160° centre-line of bottom impeller Spare probe port Lower 25.0 (0.98″) 1.) 913 (36″) In the plane of (DOT-type) ring 2.) 150° centre-line of bottom impeller Spare probe port Lower 38.1 (1.5″) 1.) 913 (36″) In the plane of (pH-type) ring 2.) 10° centre-line of bottom impeller Sample valve Lower 12.7 (0.5″) 1.) 913 (36″) NovAseptic (main) ring 2.) 40° type Sample valve Lower 12.7 (0.5″) 1.) 913 (36″) NovAseptic (backup) ring 2.) 50° type Alkali addition 1 - Lower 50.8 (2″) 1.) 913 (36″) Diametrically Tank 1 ring 2.) 190° opposite pH probes Alkali addition 2 - Centre-line 50.8 (2″) 1.) 2411 (95″) Diametrically Tank 1 of upper 2.) 190° opposite pH impeller probes Continuous feed 1 - Lower 50.8 (2″) 1.) 913 (36″) Diametrically Tank 2 ring 2.) 200° opposite pH probes Continuous feed 2 - Lower 50.8 (2″) 1.) 913 (36″) Diametrically Tank 3 ring 2.) 210° opposite pH probes DOT control sparger N/A 101.6 (4″) 1.) 593 (23″) Diametrically orifice 2.) 0° opposite ballast sparger Ballast sparger N/A 101.6 (4″) 1.) 593 (23″) Diametrically orifice 2.) 180° opposite control sparger Overlay gas Head 101.6 (4″) 1.) N/A Diametrically plate 2.) 135° opposite vent out Exhaust Impeller 50.8 (2″) 1.) N/A Diametrically vent out flange 2.) 315° opposite over- plate lay gas in Harvest Base 76.2 (3.0″) 1.) N/A NovAseptic valve plate 2.) Centre type to allow free draining Antifoam Head 50.8 (2″) 1.) N/A Liquid addition plate 2.) 170° surface/0.25T from tank centre Shot feed 1 - Head 50.8 (2″) 1.) N/A Liquid surface LS1 plate 2.) 190° Shot feed 2 - Glu- Head 50.8 (2″) 1.) N/A Liquid surface cose shot plate 2.) 180° Small add - Head 50.8 (2″) 1.) N/A Liquid surface - Spare plate 2.) 200° directed into vessel wall Media Head 101.6 (4″) 1.) N/A Nozzle directed inlet plate 2.) 310° onto vessel wall Inoculum transfer Head 101.6 (4″) 3.) N/A Nozzle directed from 4000 L to plate 4.) 320° onto vessel wall 20000 L CIP - Impeller 76.2 (3″) 1.) N/A CIP'ing of Spray ball flange 2.) 270° highest point plate CIP - Head 76.2 (3″) 5.) N/A As per CIP Spray ball plate 6.) 60° design CIP - Head 76.2 (3″) 1.) N/A As per CIP Spray ball plate 2.) 180° design CIP - Head 76.2 (3″) 1.) N/A As per CIP Spray ball plate 2.) 300° design Pressure indicating Head 38.1 (1.5″) 1.) N/A As per vessel transmitter (PIT) plate 2.) 60° vendor design Pressure Head 38.1 (1.5″) 1.) N/A As per vessel gauge plate 2.) 50° vendor design Rupture Head 101.6 (4″) 1.) N/A As per vessel disc plate 2.) 280° vendor design Spare Head 101.6 (4″) 1.) N/A As per vessel nozzle plate 2.) 160° vendor design Sight Head 101.6 (4″) 1.) N/A As per vessel glass plate 2.) 70° vendor design Light Head 76.2 (3″) 1.) N/A As per vessel glass plate 2.) 75° vendor design Manway Head 457.2 (18″) 1.) N/A Personnel plate 2.) 90° entry Agitator head/ Head 1320.8 (52″) N/A Entry/removal flange plate Impeller shaft Agitator 304.8 (12″) N/A Entry/removal and seal head/ manway flange ¹Measured from the tangential line of the base plate. Degrees pertain to plane of clockwise rotation. ²Diameter of nozzle at bioreactor. Addition Ports, Surface and Sub-Surface

The need to determine addition ports that terminate at liquid surface and those that are subsurface was determined by operational scenarios and the effects of feed strategy on process control.

Currently the protein free process has two continuous feeds that need to be discharged in well-mixed area of the bioreactor. Additional provision for glucose and an “LS1-type” shot addition is also integrated in the well mixed region. The foam is controlled by surface addition of 1 in 10 diluted C-emulsion. The inoculation of seed into the pre-inoculation bioreactor is served by avoiding build up of foam which will arise as the culture is dropped onto the surface of the medium. Following ports were designed:

-   -   Six surface additions with media inlet, inoculum inlet, one         small addition inlet directed into the wall of the vessel while         the others dropped onto the liquid surface away from the tank         wall.     -   Four subsurface additions comprised of inlets from the two feed         tanks and bi-level inlet from the alkali tank.         Sample Ports

The sample port design allows a representative sample to be taken from the bioreactor. Therefore any residual material must be as small as possible. The samples taken are used to determine off line checks for dissolved gases, pH, nutrients and biomass concentration. The orifice of the port opening is large enough to prevent sieving causing biomass aggregates to be retained. The 2 mm orifice NovaSeptum sampling device was used. However this has to be balanced with the desire to keep residual volume of the port low. The port needs to be positioned in a well-mixed zone adjacent to the probes that need to be verified by off-line checks and will be determined via nozzle position (see table 10).

Add Tanks

In order to reduce cost and time the add-tanks supplying the bioreactor are of modular design. The production bioreactor has three litre nominal volumes add tanks. The add tanks are filled at 25 I/min. The flow rate of feeds from the add tanks to the bioreactor is controlled at 0.2 to 1.0 millilitres of feed per litre of post-inoculation bioreactor volume per hour (ml/l/h). It is expected that feed rate is controlled at ±5% of set point.

The production bioreactor is serviced by three 1372 mm ID by 1880 mm add tanks. These tanks have the capability to be cleaned and sterilised independently and together with the production bioreactor.

Manway

Access into the bioreactor is required for certain service operations. Access can be gained by considering a flanged head plate or incorporation of a manway into the head plate. The need for access into the bioreactor is for:

-   -   Installation of impellers.     -   Installation and replacing of impeller and impeller shaft.     -   Installation and replacing of mechanical seal.     -   Service of vessel furniture.     -   Potential modification of sparger position to obtain desired         hydrodynamic characteristics.

The size of the manway must be sufficient to allow access for the above objectives. The manway used was of sufficient diameter to allow the removal of two impellers of 1219 mm diameter.

Volume Measurement

The design ensures that any sensor gives sufficient precision in volume measurement around the operating range.

The volume measurement in bioreactor is able to measure a range of 13 000 to 25 000 litres. The sensor sensitivity needs to be at least 0.5% of full span.

Volume measurement in the feed add-tanks and alkali tank is able to measure 0 to 2200 and 2500 litres respectively. The sensor sensitivity needs to be at least 0.2% of full span. This will permit hourly verification of feed flow rate at the minimum flow rate of 0.2 ml/l per hour or 3.5 l per hour by measuring the volume decrease in the add tanks.

Bioreactor Temperature Control

The medium is brought to operating temperature and pH by process control. This is achieved by “gentle” heating of the jacket (avoid high temperature at vessel wall). The temperature control range during operation is 36 to 38° C. with an accuracy of ±0.2° C. at set point.

Jacket

The bioreactor jacket area is specified with the following considerations in mind: —

-   -   Steam sterilisation at 121-125° C.     -   Warming up of medium from 10° C. to 36.5° C. in <2 h.     -   All points within the bioreactor must reach ±0.2° C. of set         point, typically 36.5° C., as measured by thermocouples.     -   Chilling of medium from 36.5° C. to 10° C. in <2 h.         Bioreactor pH Control

The process pH is monitored and controlled with probes connected via a transmitter to a DCS based process controller. The process is be controlled by addition of CO₂ to bring the pH down to set point and addition of alkali to bring pH up to set point. pH is controlled at ±0.03 of set point.

Alkali is added through two addition points to distribute the alkali. This ensures quicker blending of alkali in the event of long recirculation time in the tank. The CO₂ is added via the control sparger.

Control and back-up probes are located in the lower port ring at 913 mm (see table 10) from tank bottom. Additionally the pH probes are located diametrically opposite the alkali addition points into the bioreactor.

Bioreactor DOT Control

Dissolved oxygen is monitored and controlled with polarographic DOT probe. The DOT set point maintained by sparging:

-   -   Initial N₂ ballast and/or air on demand.     -   Air ballast with air on demand.     -   Air ballast with oxygen on demand.     -   Reversing gas usage once oxygen demand decreases.

Cascade DOT control allows DOT set point to be maintained through changes in the ballast and demand gas in conjunction with ramping of agitator speed.

In order to control pCO₂ the ballast required to strip out excess dCO₂ impacts DOT control. Therefore the DOT control is considered together with pCO₂ control for those processes where metabolic CO₂ is liberated. DOT is controlled at ±2% of set point. Control and back-up probes are located in the lower port ring at 913 mm from tank bottom.

Bioreactor Dissolved CO₂ Control

The process dCO₂ is monitored with an pCO₂ probe and excess dCO₂ is stripped by gassing CA through the ballast sparger. The optimal position for this probe is close to the pH probes.

Feed Addition Control

The feeds (SF22 and amino acid) are high in pH and osmolality. Therefore bolus additions need to be avoided to maintain good pH control. However the control of desired flow rates (±5% of set point) is technically challenging. Therefore an addition strategy that encompasses point of addition with delivery mode avoids the circulation of feed bolus and potential variations of pH control.

Therefore the point of addition is in the plane of the centre-line of the bottom impeller that is 913 mm from tank bottom to assist in the rapid blending of feed bolus.

Antifoam Addition Control

Antifoam (C-emulsion) addition is added as required to maintain the bioreactor liquid surface free of foam. A working stock of 1 in 10 diluted C-emulsion can be dosed on the liquid surface. The antifoam suspension is continuously agitated in the storage container to prevent partitioning. It is important to dose the antifoam close to the centre of the tank to diminish the effects of the radial component of the fluid flow carrying the antifoam to the tank walls where it will adhere. Therefore the addition point is 0.25×T toward the tank centre or 699 mm from tank centre.

EXAMPLE 2: 4000 LITRE BIOREACTOR

Vessel Geometry

The vessel geometry for the 4000 litre bioreactor was determined by an iterative design basis in which the maximum working volume, freeboard straight side distance aspect ratio (H_(L)/T) and impeller to tank diameter ratio (D/T) are altered until an acceptable aspect ratio is achieved.

Bioreactor Aspect Ratio H_(L)/T

Table 11 describes the aspect ratios in the 4000 litre bioreactor at the various operating volumes during normal processing. These aspect ratios arise from the selection of tank ID and the operating volume required. From a processing perspective the mixing requirements at the three operating conditions are different. During pre-inoculation stage the bioreactor mixing is important to allow medium to equilibrate with minimal K_(L)a requirement. However for post-inoculation and pre-transfer stages both mixing and K_(L)a are important considerations. Therefore both these features were tested at the aspect ratio range.

TABLE 11 Key operating volumes and aspect ratios in the 4000 liter bioreactor Volume, Liquid head, Aspect ratio, L mm H_(L)/T Pre-Inoculation 1.) 1914 1.) 1031 1.) 0.63 2.) 2782 2.) 1448 2.) 0.89 3.) 3077 3.) 1590 3.) 0.98 Post Inoculation & 1.) 2153 1.) 1146 1.) 0.70 Pre-transfer 2.) 3478 2.) 1783 2.) 1.10 3.) 3846 3.) 1960 3.) 1.21 Tank Diameter

The tank diameter was altered to obtain the optimal aspect ratio H_(L)/T. Changes to tank internal diameter are limited by acceptable aspect ratio and plant footprint. The tank ID is 1626 mm.

Tank Height

Tank height is determined from the maximum operating volume, aspect ratio H_(L)/T, freeboard straight side length, base and top plate design. The final tank height is a compromise value determined from volumetric contingency for foam, plant height and acceptable impeller shaft length. The head to base tan line height is 2817 mm.

Freeboard Height

The freeboard height of 500 mm (1039 litre or 27% v/v of the maximum operating volume) is used for this seed bioreactor.

Head and Base Plate

The base and head plate design is a ASME F&D design for this seed bioreactor.

Bioreactor Agitation Requirement

The agitation of the bioreactor is to achieve rapid mixing, maintain homogeneity, maintain mammalian cells in suspension and gas bubble dispersion. The underlying issue for achieving the above objectives is minimising cell damage through shear forces originating from impeller geometry and eddies or vortices created behind the impeller blades. A compromise of the above objectives was achieved by selection of an appropriate impeller type.

Bottom Versus Top Driven Impeller Shaft

The motor drive is top mounted for the benefits already highlighted.

Baffles

The baffle requirement for centre mounted impeller is critical to prevent vortex formation. The critical issues related to baffles are baffle number, baffle width (W), baffle length (H_(baffle)) and baffle to tank wall clearance (W_(c)).

Four equally spaced baffles that are 0.1×T or 163 mm wide 1.1×H−H_(h) or 2195 mm tall and have a baffle to tank wall clearance, W_(c) of 0.01×T or 16 mm were used.

The thickness of the baffles is not specified but the thickness needs to ensure rigidity to the radial component of the fluid flow. Additionally thickness needs to ensure the baffle plates are not warped during SIP thereby affecting the baffle to tank wall clearance.

Impeller Type, Size and Number

The impellers for this bioreactor are identically formed to the 20000 litre vessel and have a identical D/T ratio of 0.44. The bottom impeller is a Lightnin's A315 at 710 mm of diameter and the top impeller is a Lightnin's A310 at 710 mm of diameter.

The impeller spacing, D_(c), D_(s) and D_(n)

The impeller spacing, D_(s), between the centre-line of the top impeller and the centre-line of the lower impeller is 1.229×D_(bottom) or 872 mm. The off bottom impeller clearance, D_(c) is 0.75×D_(bottom) or 531 mm. This allows the lower impeller to remain submerged at the lowest post-inoculation volume of 2153 litres and both impellers submerged at 3367 litres with liquid head above the upper impeller (D_(o)) of 0.5×D_(top) or 358 mm.

Table 12 highlights the volumes that will form liquid surfaces or lower liquid cover above the impeller. Agitation can be modified to avoid foaming at these critical volumes.

TABLE 12 Key operating volumes that cause interaction with impellers and liquid surface Interaction Volume, L Potential Operation Submerge top impeller 3433 Volume seen during with 0.5D_(A310) liquid inoculation of 1 in 5 cover processes Liquid surface touching 2758 Volumes seen during pre- top edge of top impeller inoculation fill of 1 in 5 processes. Liquid surface touching 2621 Volumes seen during pre- bottom edge of top impeller inoculation fill of 1 in 5 processes. Submerge bottom impeller 1654 Volumes seen during pre- with 0.5D_(A315) liquid cover inoculation fill of 1 in 5 processes. Liquid surface touching 1104 Volumes seen during pre- top edge of bottom impeller inoculation fill of 1 in 5 and 1 in 9 processes. Liquid surface touching  650 Volumes seen during pre- bottom edge of bottom inoculation fill of 1 in 5 and 1 impeller in 9 processes. ⁽¹⁾ Minimum operating volume with lower impeller submerged is 1654 liters and minimum operating volume with both impellers submerged is 3433 liters ⁽²⁾ The operating volume range is 1914 to 3846 liters.

The 4000 l bioreactor can operate at two discrete post-inoculation volumes with either the lower impeller submerged (during cultivation of 1 in 9 seeding process) or with both impellers submerged (during 1 in 5 seeding process), table 13 shows the liquid cover obtained for the upper and lower impeller during its operation.

A liquid cover of 0.67 to 0.82×D_(bottom) above the lower A315 impeller is observed during cultivation of the 1 in 9 seeded processes. This is within the recommendations of 0.5 to 1×D.

A liquid cover of 0.06 to 0.78×D_(top) above the top A310 impeller is observed during cultivation of 1 in 5 seeded process. The lower liquid cover is outside the recommendation. However this liquid cover is observed during pre-inoculation when mixing and agitation are less critical.

TABLE 13 Key operating volumes and the liquid cover above top impeller, Do and bottom impeller, D_(Bo) Cylinderical Do, D_(Bo), Do as ratio D_(Bo) as ratio Operating volume, L height, H (mm) mm mm of D_(A310) of D_(A315) Post - Inoculation and 863 or — 614 or — 0.82D_(A315) Pre-Transfer, 2153 L 34″ 24″ Post - Inoculation and 1501 or 380 or — 0.53D_(A310) — Pre-Transfer, 3478 L 59″ 15″ Post - Inoculation and 1678 or 557 or — 0.78D_(A310) — Pre-Transfer, 3846 L 66″ 22″ Pre-Inoculation, 1914 L 748 or — 499 or — 0.67D_(A315) 29″ 20″ Pre-Inoculation, 2782 L 1166 or 45 or — 0.06D_(A310) — 46″ 2″ Pre-Inoculation, 3077 L 1308 or 187 or — 0.26D_(A310) — 52″ 7″ ⁽¹⁾ Off bottom impeller clearance, Dc = 531 mm (0.75D_(A315)), Impeller separation, Ds = 872 mm (1.229D_(A315)), tank ID of 1626 mm and Height of ASME F&D base plate, H_(h) = 282 mm ⁽²⁾ Do = H − Ds − (Dc − H_(h)) and D_(Bo) = H − (Dc − H_(h)) Agitation Rate—rpm, P/V and Tip Speed

Table 14 specifies the agitation rate for the 4000 litre bioreactor. The bioreactor will be agitated typically at 20-260 W/m⁻³, preferably at 55-85 W/m⁻³. The agitation strategy was developed during the 500 litre pilot fermentations. An agitation rate of 0 to 88±1 rpm is therefore used as an operational range.

TABLE 14 Agitation rate for the 4000 L bioreactor Agitation rate, Power per unit volume, Tip Speed, rpm W/m3 m/s ¹0-88 0-150 0.0-3.3 ²0-86 0-150 0.0-3.2 ¹When both impellers submerged ²When bottom impeller submerged Mechanical Seals Specification

A double mechanical seal that is condensate lubricated is used as described.

Bioreactor Aeration Requirement

Table 15 shows the gas flow, based upon scale up of constant superficial gas velocity, for DOT and pH control during the inoculum expansion in the 4000 litre bioreactor. Oxygen is not required for DOT control. However oxygen enriched air can be used to facilitate lower gassing to prevent excess foaming. It is recommended that a smaller range N₂ MFC should supply nitrogen for early DOT control and reducing deviant, high levels of DOT.

TABLE 15 Gas flow rate and MFC operating ranges for the 4000 liter bioreactor Gas Operating range Comments Head Space¹ 1. Clean air 1. 0-200 SLPM 1. Head space purging of CO₂ and O₂ 2. Nitrogen 2. Utility rated 2. For rapid DOT probe zeroing 3. Helium 3. Utility rated 3. Tank integrity testing Control Sparger 1. Clean air¹ 1. 10-60 SLPM 1. Gas flow under DOT control 2. Oxygen 2. 1.0-10 SLPM 2. Gas flow under DOT control 3. Carbon dioxide 3. 1.0-20 SLPM 3. Gas flow under pH control 4. Nitrogen² 4. 2.0-15 SLPM 4. Early DOT control by ballast 5. Helium 5. Utility rated 5. Tank integrity testing ¹The air and nitrogen gas flow into bioreactor via a bypass for post SIP tank pressurisation. ²Nitrogen delivered via the 2 to 15 SLPM N₂ MFC and could be used during early DOT control

The calculation of hole size and number of holes, for the fluted sparger, is iterated until the target Reynolds number of gas emerging from holes is <2000 and the Sauter mean bubble diameter for a bubble chain regime is approximately 10 mm.

Table 16 show the key sparger design specification for the 4000 litre bioreactor. The sparger length, S_(L) of 568 mm is determined for pipe geometry. The holes are distributed on either end of the sparger to prevent bubble liberating directly under the A315 hub. Alternatively a crescent geometry can be used. The pipe diameter is selected to aid spacing of the desired number of holes. The diameter is 38 mm. The mm holes are located on the dorsal surface of the sparger with a single 2 mm hole located on the ventral surface to aid free CIP drainage of the sparger.

The bioreactor port for sparger installation is designed to a fit pipe design of diameter of 38 mm. The position of the port allows the placement of a control sparger at a distance of 194 mm, D_(c)−S_(c) below the bottom edge of the lower impeller and no greater 337 mm from tank bottom, S_(c).

TABLE 16 Design specification for the 4000 liter bioreactor sparger Control Parameter Sparger Gas flow, SLPM 105 Number of sparge holes 100 Orifice diameter, d_(o), m 0.002 Gas flow, m³ · s⁻¹ 1.75E−03 Orifice area, m² 3.14E−06 Total orifice area, m² 3.14E−04 Density of air, Kg · m⁻³ 1.166 Viscosity, Nm · s⁻² 1.85E−05 Sauter mean diameter, mm (d_(vs) = 1.17 V_(o) ^(0.4) d_(o) ^(0.8) g^(−0.2)) 10.21 Gravitional acceleration, g, m · s⁻² 9.807 Density difference, Kg · m⁻³ 1048.834 Reynold's number, >2000 jetting regime 704 Gas velocity at sparger, V_(o), m/s 5.57 Sparger length, S_(L), m 0.568 Combined length to drill required holes, m 0.2 Number of rows to fit required holes in length S_(L) 1 Sparger to tank bottom clearance, Sc, m 0.337 (13″) Sparger to bottom impeller clearance, Dc-Sc, m 0.194 (8″)  Position of Probes, Addition and Sample Ports

The design basis for positioning of probes, addition and sample ports has been covered in example 1 and are listed in table 17:

TABLE 17 Probe, addition and sampling port specification for the 4000 liter bioreactor ²Diameter, ¹Position, Probe/Port Location mm (inches) mm (inches) Rational Temperature Lower 38.1 (1.5″) 1.) 531 (21″) In the plane of (main) ring 2.) 30° centre-line of bottom impeller Temperature Lower 38.1 (1.5″) 1.) 531 (21″) In the plane of (backup) ring 2.) 170° centre-line of bottom impeller pH Lower 38.1 (1.5″) 1.) 531 (21″) In the plane of (main) ring 2.) 10° centre-line of bottom impeller pH Lower 38.1 (1.5″) 1.) 531 (21″) In the plane of (backup) ring 2.) 20° centre-line of bottom impeller DOT Lower 25.0 (0.98″) 1.) 531 (21″) In the plane of (main) ring 2.) 150° centre-line of bottom impeller DOT Lower 25.0 (0.98″) 1.) 531 (21″) In the plane of (backup) ring 2.) 160° centre-line of bottom impeller Spare-1 Lower 25.0 (0.98″) 1.) 531 (21″) In the plane of (nutrient) ring 2.) 170° centre-line of bottom impeller Spare-2 Lower 38.1 (1.5″) 1.) 531 (21″) In the plane of (pCO₂) ring 2.) 180° centre-line of bottom impeller Spare-3 Lower 50.8 (2″) 1.) 531 (21″) In the plane of (biomass) ring 2.) 190° centre-line of bottom impeller Sample valve Lower 12.7 (0.5″) 1.) 531 (21″) NovAseptic type (main) ring 2.) 40° Alkali Lower 50.8 (2″) 1.) 531 (21″) Diametrically addition ring 2.) 190° opposite pH probes Feed 1 Lower 50.8 (2″) 1.) 531 (21″) Diametrically ring 2.) 200° opposite pH probes Feed 2 Lower 50.8 (2″) 1.) 531 (21″) Diametrically ring 2.) 210° opposite pH probes Antifoam Head 50.8 (2″) 1.) N/A Liquid surface/ addition plate 2.) 170° 0.25T from tank centre Spare surface Head 50.8 (2″) 3.) N/A Liquid surface addition plate 4.) 180° directed to vessel wall DOT control N/A 50.8 (2″) 337 (13″) sparger orifice 1.) 0° Overlay Head 101.6 (4″) 1.) N/A Diametrically gas plate 2.) 135° opposite vent out Exhaust Head 50.8 (2″) 1.) N/A Diametrically vent out plate 2.) 315° opposite overlay gas in Transfer Base 76.2 (3.0″) 1.) N/A NovAseptic type valve plate 2.) Centre to allow free draining Inoculum transfer Head 101.6 (4″) 1.) N/A Directed into from 1000 L to 4000 L plate 2.) 320° vessel wall Media Head 101.6 (4″) 3.) N/A Directed into inlet plate 4.) 310° vessel wall CIP - Impeller 76.2 (3″) 1.) N/A CIP'ing of Spray ball flange plate 2.) 270° highest point CIP - Head 76.2 (3″) 1.) N/A Spray ball plate 2.) 60° CIP - Head 76.2 (3″) 1.) N/A Spray ball plate 2.) 180° CIP - Head 76.2 (3″) 1.) N/A Spray ball plate 2.) 300° Pressure indicating Head 38.1 (1.5″) 1.) N/A transmitter (PIT) plate 2.) 60° Pressure Head 38.1 (1.5″) 1.) N/A gauge plate 2.) 50° Rupture Head 101.6 (4″) 1.) N/A disc plate 2.) 280° Spare Head 101.6 (4″) 1.) N/A nozzle plate 2.) 160° Sight Head 101.6 (4″) 1.) N/A glass plate 2.) 70° Light Head 76.2 (3″) 1.) N/A glass plate 2.) 75° Agitator Head 813 (32″) N/A Entry/removal head/flange plate Impeller shaft and Agitator 152 (6″) N/A Entry/removal seal manway head/flange ¹Measured from the tangential line of the base plate. Degrees pertain to plane of clockwise rotation. ²Diameter of nozzle at bioreactor. Addition Ports, Surface and Sub-Surface

The need to categorise additions ports that terminate at liquid surface and those that are subsurface is determined by the operational scenarios and effects of feed strategy on process control.

The 4000 litre bioreactor has been designed to accept two subsurface feeds and alkali that need to be discharged in well-mixed area of the bioreactor. The foam is controlled by surface addition of 1 in diluted C-emulsion. A single spare above surface addition port directed to the vessel wall is also designed for future flexibility. The splashing of culture onto the surface of the medium during inoculation of the seed bioreactor can be avoided to prevent build up of foam. Therefore the inoculum addition port is above surface and directed to the vessel wall. The use of the harvest port in the base plate is the ideal port for removal of inoculum during transfer of inoculum. Additionally the medium addition port is directed to the vessel wall. In summary the total addition ports are:

-   -   Four surface additions with medium inlet, inoculum inlet and a         spare small addition directed to the vessel wall and addition         port for antifoam dropped on to the liquid surface away from the         vessel wall.     -   Three subsurface additions for feeds and alkali.         Sample Ports

The sample port design is similar to that specified for the 20 000 litre bioreactor.

Volume Measurement

The level sensor is able to measure up to 4000 litres with an accuracy ±0.5% of full span.

Bioreactor Temperature Control

The 1914 to 3077 litre of medium are brought to operating temperature, typically 36.5° C. by process control. This is achieved by “gentle” heating of the jacket and avoid high temperature at vessel wall.

Jacket

The bioreactor jacket area is specified with the following considerations in mind:

-   -   Steam sterilisation at 121-125° C.     -   Warming up of 1914-3077 litres of medium from 10° C. to 36.5° C.         in <2 h.     -   All points within the bioreactor must reach ±0.2° C. of set         point, typically 36.5° C. as measured by thermocouples.     -   Chilling of 1914-3077 litres of medium from 36±2° C. to 10° C.         in h.         Bioreactor pH Control

The process pH is monitored and controlled with probes connected via a transmitter to a DCS based process controller. The process pH is controlled by addition of CO₂ through the control sparger to bring the pH down to set point and addition of alkali to bring pH up to set point.

Alkali is added through at least one subsurface port at centre-line of the bottom impeller. The CO₂ will be added via the control sparger.

Control and backup probes are in the lower port ring at 531 mm from tank bottom as shown in table 17.

Bioreactor DOT Control

Dissolved oxygen is monitored and controlled with polarographic DOT probe. The DOT set point maintained by sparging:

-   -   Initial N₂ ballast and/or air on demand.     -   Air ballast with air on demand.     -   Air ballast with oxygen on demand.

DOT control allows DOT set point to be maintained through interchangeable use of oxygen or air as demand gas. It is not envisaged that pCO₂ control is required in the inoculum bioreactor. Control and backup probes are in the lower port ring at 531 mm from tank bottom as shown in table 17.

Feed Addition Control

The point of addition is 531 mm from tank bottom, in the plane of the centre-line of the lower impeller to assist in the rapid dissipation of feed bolus.

Antifoam Addition Control

The addition point is at surface projecting 0.25×T toward the tank centre or 407 mm from centre of tank.

EXAMPLE 3: 1000 LITRE BIOREACTOR SPECIFICATION

Vessel Geometry

The vessel geometry for the 1000 litre bioreactor was determined by an iterative design basis in which the maximum working volume, freeboard straight side distance, aspect ratio (H_(L)/T) and impeller to tank diameter ratio (D/T) are altered until an acceptable aspect ratio is achieved.

Bioreactor Aspect Ratio H_(L)/T

Table 18 below describes the aspect ratios in the 1000 litre bioreactor at various operating volumes during normal processing. These aspect ratios arise from the selection of tank ID and the operating volume required. From a processing perspective the mixing requirements at the different operating conditions are different. During pre-inoculation stage the bioreactor mixing is important to allow medium to equilibrate with minimal K_(L)a requirement. However with post-inoculation and pre-transfer stages both mixing and K_(L)a are important considerations. Therefore both of these features were tested at the aspect ratio range.

TABLE 18 Key operating volumes and aspect ratios in the 1000 liter bioreactor Volume, Liquid head, Aspect ratio, L mm H_(L)/T Stage N-3 Pre-Inoculation 250 484 0.56 Stage N-3 Post Inoculation & 300 570 0.66 Pre-transfer/Harvest Stage N-2 Pre-Inoculation, Post- 1.) 400¹ 1.) 740 1.) 0.86 drain Pre-refill 2.) 50-100¹ 2.) 143-228 2.) 0.17-0.26 3.) 192² 3.) 385 3.) 0.45 Stage N-2 Post Inoculation & 1.) 450 1.) 826 1.) 0.96 Pre-transfer/Harvest 2.) 450-900³ 2.) 826-1594 2.) 0.96-1.84 3.) 960⁴ 3.) 1696 3.) 1.96 ¹Pre-inoculation volume and rolling seed inoculation volume for the 1 in 9 sub-cultivation process. ²Rolling seed inoculation volume for the 1 in 5 sub-cultivation processes. ³Rolling seed post inoculation & pre-transfer volume for the 1 in 9 sub-cultivation processes. ⁴Rolling seed post inoculation & pre-transfer volume for the 1 in 5 sub-cultivation processes. Tank Diameter

The tank diameter is altered to obtain the optimal aspect ratio H_(L)/T. Changes to tank internal diameter are limited by acceptable aspect ratio and plant footprint. The tank ID is 0.864 m.

Tank Height

The tank height is determined from the maximum operating volume, aspect ratio H_(L)/T, freeboard straight side length, base and top plate design. The final tank height is a compromise value determined from volumetric contingency for foam, plant height and acceptable impeller shaft length. The head to base tangent line height is 2.347 m.

Freeboard Height

The freeboard height of 500 mm (293 litres or 31% v/v of the maximum operating volume) is used for this seed bioreactor.

Head and Base Plate

The base and head plate design is ASME F&D for this seed bioreactor.

Bioreactor Agitation Requirement

Agitation of the bioreactor is to achieve rapid mixing, maintain homogeneity, maintain mammalian cells in suspension and gas bubble dispersion. The underlying issue with achieving the above objectives is to minimise cell damage through shear forces originating from impeller geometry and “eddies” or vortices created behind the impeller blades. A compromise of the above objectives was achieved by selection of an appropriate impeller type and gassing strategy.

Bottom Versus Top Driven Shaft

The motor drive is top mounted for the benefits as already highlighted.

Baffles

The baffle requirement for a centre mounted impeller is critical to prevent vortex formation. The critical issues related to baffles are baffle number, baffle width (W), baffle length (H_(baffle)) and baffle to tank wall clearance (W_(c)).

Four equally spaced baffles that are 0.1×T or 86 mm wide 1.1×H−H_(h) or 2099 mm tall and have a baffle to tank wall clearance, W_(c) of 0.01×T or 9 mm were used.

The thickness of baffle is not specified but the thickness needs to ensure rigidity to the radial component of the fluid flow. Additionally thickness needs to ensure the baffle plates are not warped during SIP thereby affecting the baffle to tank wall clearance.

Impeller Type, Size and Number

The impellers for the 1000 l bioreactor should be identical formed to the 20 000 litre vessel with an identical D/T ratio. Therefore the bottom impeller is a Lightnin's A315 at 381 mm diameter and the top impeller is a Lightnin's A310 at 381 mm diameter.

The impeller spacing, D_(c), D_(s) and D_(o)

The impeller spacing (D_(s)) between the centre-line of the top impeller and the centre-line of the bottom impeller is 2×D_(bottom) (762 mm). The off bottom impeller clearance (D_(o)) is 0.4×D_(bottom)(152 mm). This allows the bottom impeller to remain submerged with liquid cover (Do) of 0.5×D_(bottom) or 190 mm at the lowest post-inoculation volume of litres and both impeller submerged at 616 litres with liquid head above the upper impeller, D_(o), of 0.5×D_(top) (190 mm).

Table 19 highlights volumes that will form liquid surfaces or lower liquid cover above the impeller Agitation can be modified to avoid foaming at these critical volumes.

TABLE 19 Key operating volumes that cause interaction with impellers and liquid surface Interaction Volume, L Potential Operation Submerge top impeller with 616 Volume seen during inoculation 0.5D_(A310) liquid cover of 1 in 5 processes and rolling operation of the 1 in 9 process Liquid surface touching top 512 Volume seen during inoculation edge of top impeller of 1 in 5 processes and rolling operation of the 1 in 9 process Liquid surface touching 492 Volume seen during inoculation bottom edge of top impeller of 1 in 5 processes and rolling operation of the 1 in 9 process Submerge bottom impeller 167 Volume seen during inoculation with 0.5D_(A315) liquid cover of 1 in 5 processes and rolling operation of the 1 in 9 process Liquid surface touching top  90 Volume seen during rolling edge of lower impeller operation of the 1 in 9 process Liquid surface touching  21 Volumes seen during pre- bottom edge of lower inoculation fill of 1 in 5 and 1 impeller in 9 processes.

The 1000 l bioreactor operates at two discrete post-inoculation volumes with either the bottom impeller submerged during the 1 in 5 processes and 1 in 9 processes or with both impellers submerged during the N-2 phase of the 1 in 5 process and rolling seed operations for both 1 in 5 and 1 in 9 processes.

Table 20 shows the liquid cover above the upper and lower impeller during operation of the 1 in 5 and 1 in 9 sub-cultivation processes. During rolling operation of the 1 in 5 and 1 in 9 processes the liquid cover above the lower impeller falls below 0.5×D. It is therefore important to reduce the agitation rate, to avoid surface gas entrainment, whilst operating at this low volume. At 960 litres a liquid cover, (D_(o)) of 2.05×Dtop is obtained. At this level K_(L)a has been shown not to be adversely affected and bulk blending is not an issue.

TABLE 20 Key operating volumes and the liquid cover above top impeller, Do and bottom impeller, D_(Bo) Cylinderical Do, D_(Bo), Do as ratio D_(Bo) as ratio Operating volume, L height, H (mm) mm mm of D_(A310) of D_(A315) Pre-Inoculation, 250 L 334 — 332 — 0.87D_(A315) Pre-Inoculation, 400 L 590 — 588 — 1.54D_(A315) Post - Inoculation and 419 — 417 — 1.10D_(A315) Pre-Transfer, 300 L Post - Inoculation and 675 — 673 — 1.77D_(A315) Pre-Transfer, 450 L Post drain, pre-bulk 235 — 233 — 0.61D_(A315) 192 L Post drain, pre-bulk 50-100 L 0-78 —  76 — 0.2D_(A315) Post - Inoculation and 1443 679 — 1.78D_(A310) — Pre-Transfer, 900 L Post - Inoculation and 1545 782 — 2.05D_(A310) — Pre-Transfer, 960 L ⁽¹⁾ Off bottom impeller clearance, Dc = 152 mm (0.4D_(A315)), Impeller separation, Ds = 762 mm (2D_(A315)), tank ID of 864 mm and Height of ASME F&D base plate, H_(h) = 151 mm ⁽²⁾ Do = H − Ds − (Dc − H_(h)) and D_(Bo) = H − (Dc − H_(h)) Agitation Rate—rpm, P/V and Tip Speed

Table 21 specifies the agitation rate for the 1000 litre bioreactor. The bioreactor is agitated at around 20-260 W/m³, preferably at 55-85 W/m³. The agitation strategy was developed during the 500 litre pilot fermentations. An agitation rate of up to 155±1 rpm is used as an operational range.

TABLE 21 Agitation rate for the 1000 L bioreactor Agitation rate, Power per unit volume, Tip Speed, rpm W · m⁻³ m · s⁻¹ ¹0-155 0-150 3.1 ²0-145 0-145 2.9 ¹When both impellers submerged ²When bottom impeller submerged Mechanical Seals Specification

A double mechanical seal that is condensate lubricated as described was used.

Bioreactor Aeration Requirement

Table 22 shows the gas flows based upon scale up of constant superficial gas velocity, for DOT and pH control during the inoculum expansion in the 1000 litre bioreactor. Oxygen will not be required for DOT control. However oxygen enriched air may be used to facilitate lower gassing to prevent excess foaming. It is recommended that the smaller range CA MFC should be used to delivery nitrogen for early DOT control and reducing deviant, high levels of DOT.

TABLE 22 Gas flow rate and MFC operating ranges for the 1000 liter bioreactor Gas Operating range Comments Head Space¹ 1. Clean air 1. 0-50 SLPM 1. Head space purging of CO₂ and O₂ 2. Nitrogen 2. Utility rated 2. For rapid DOT probe zeroing 3. Helium 3. Utility rated 3. Tank integrity testing Control Sparger 1. Clean air¹ 1. 2-20 SLPM 1. Gas flow under DOT control 2. Oxygen 2. 0.2-5 SLPM 2. Gas flow under DOT control 3. Carbon dioxide 3. 0.2-10 SLPM 3. Gas flow under pH control 4. Nitrogen² 4. 0.2-5 SLPM 4. Early DOT control by ballast 5. Helium 5. Utility rated 5. Tank integrity testing ¹The air and nitrogen gas flow into bioreactor via a bypass for post SIP tank pressurisation. ²Nitrogen delivered via the 0 to 5 SLPM CA MFC, could be used during early DOT control.

The calculation of hole size and number is iterated until the target Reynolds number of gas emerging from holes is <2000 and the Sauter mean bubble diameter for a bubble chain regime is approximately 10 mm.

Table 23 shows the key sparger design specification for the 1000 litre bioreactor. The sparger length, S_(L) of 305 mm is determined for pipe geometry. The holes are distributed on either end of the sparger to prevent bubble liberating directly under the A315 hub. Alternatively a crescent geometry can be considered.

The pipe diameter is 25 mm. 30 2 mm holes are located on the dorsal surface of the sparger with a single 2 mm hole located on the ventral surface to aid free CIP drainage of the sparger.

The bioreactor port for sparger installation is designed to fit pipe design of diameter of 25 mm. The position of port allows the placement of control sparger at a distance of 88 mm (D_(c)−S_(c)) below the bottom edge of the bottom impeller and no greater than 64 mm from tank bottom (S_(c)).

TABLE 23 Design specification for 1000 liter bioreactor spargers Control Parameter Sparger Gas flow, SLPM 35 Number of sparge holes 30 Orifice diameter, d_(o), m 0.002 Gas flow, m³ · s⁻¹ 5.83E−04 Orifice area, m² 3.14E−06 Total orifice area, m² 9.42E−05 Density of air, Kg · m⁻³ 1.166 Viscosity, Nm · s⁻² 1.85E−05 Sauter mean diameter, mm (d_(vs) = 1.17 V_(o) ^(0.4) d_(o) ^(0.8) g^(−0.2)) 10.65 Gravitional acceleration, g, g m · s⁻² 9.807 Density difference, Kg · m⁻³ 1048.834 Reynold's number, >2000 jetting regime 782 Gas velocity at sparger, V_(o), m/s 6.19 Sparger length, S_(L), m 0.305 Combined length to drill required holes, m 0.06 Number of rows to fit required holes in length S_(L), m 1 Sparger to tank bottom clearance, Sc, m 0.064 Sparger to bottom impeller clearance, Dc-Sc, m 0.088 Position of Probes, Addition and Sample Ports

The design basis for positioning of probes, addition and sample ports is the same as for the 20 000 l bioreactor.

TABLE 24 Probe, addition and sampling port specification for the 1000 liter bio-reactor Diameter, ¹Position, Probe/Port Location mm (inches) mm (inches) Rational Temperature Lower 38.1 (1.5″) 1. 286 (11″) Positioned to (main) ring 2. 30° minimise monitored volume Temperature Lower 38.1 (1.5″) 1. 286 (11″) Positioned to (backup) ring 2. 170° minimise monitored volume PH Lower 38.1 (1.5″) 1. 286 (11″) Positioned to (main) ring 2. 10° minimise monitored volume PH Lower 38.1 (1.5″) 1. 286 (11″) Positioned to (backup) ring 2. 20° minimise monitored volume DOT Lower 25.0 (0.98″) 1. 286 (11″) Positioned to (main) ring 2. 150° minimise monitored volume DOT Lower 25.0 (0.98″) 1. 286 (11″) Positioned to (backup) ring 2. 160° minimise monitored volume Spare-2 Lower 38.1 (1.5″) 1. 286 (11″) Positioned to (spare - ring 2. 180° minimise monitored pCO₂) volume Spare-3 Lower 50.8 (2″) 1. 286 (11″) Positioned to (spare - ring 2. 190° minimise monitored Biomass) volume Sample valve Lower 38.1 (1.5″) 1. 286 (11″) NovAseptic (main) ring 2. 40° type Sample valve Lower 38.1 (1.5″) 1. 286 (11″) NovAseptic (back up) ring 2. 40° type Alkali Lower 12.7 (0.5″) 1. 286 (11″) Diametrically addition ring 2. 190° opposite pH probes Feed 1 Lower 12.7 (0.5″) 1. 286 (11″) Diametrically ring 2. 200° opposite pH probes Feed 2 Lower 12.7 (0.5″) 1. 286 (11″) Diametrically ring 2. 210° opposite pH probes Antifoam Head 50.8 (2″) 1. N/A Liquid surface/0.25T addition plate 2. 170° from tank centre Spare surface Head 50.8 (2″) 1. N/A Liquid surface addition plate 2. 180° directed to vessel wall DOT control - N/A 50.8 (2″) 1. 64 (2.5″) sparger orifice 2. 0° Overlay Head 38.1 (1.5″) 1. N/A Diametrically gas plate 2. 135° opposite vent out Exhaust Head 38.1 (1.5″) 1. N/A Diametrically vent out plate 2. 315° opposite overlay gas in Transfer Base 50.8 (2.0″) 1. N/A NovAseptic type to valve plate 2. Centre allow free draining Media Head 76.2 (3″) 1. N/A Directed into inlet plate 2. 310° vessel wall Inoculum transfer Head 50.8 (2.0″) 1. N/A Directed into from S200 to plate 2. 320° vessel wall 1000 L CIP - Head 76.2 (3″) 1. N/A CIP'ing of Spray ball plate 2. 270° highest point CIP - Head 76.2 (3″) 1. N/A Spray ball plate 2. 60° Pressure Head 38.1 (1.5″) 1.) N/A gauge plate 2.) 50° Rupture Head 50.8 (2″) 1.) N/A disc plate 2.) 280° Spare Head 101.6 (4″) 1.) N/A nozzle plate 2.) 160° 1. Hand hole Head 1. 203.2 (8″) 1.) N/A Single port 2. Sight glass plate 2. 101.6 (4″) 2.) 70° permitting two functions Agitator shaft Head 152.4 (6″) 1.) N/A Centre of opening plate 2.) 75° head plate ¹Measured from the tangential line of the base plate. Degree pertains to plane of clockwise rotation. ⁽²⁾ Diameter of nozzle at the bioreactor

In order to monitor, control and sample from a volume of 50 l, the probes and port ring needs to be 151 mm from tank bottom. However the probe/port ring cannot be located this low as it falls on the weld of the base plate and the straight cylindrical side of the bioreactor. The probe and port ring has been specified at 286 mm from tank bottom. This permits a volume of 134 litres to be monitored, controlled and sampled. The probes/port ring is located as close to the tank bottom as permitted to minimise the monitored/controlled volume.

Addition Ports, Surface and Sub-Surface

The 1000 litre bioreactor has been designed to accept two subsurface feeds and alkali to be discharged into a well-mixed area of the bioreactor. The foam is controlled by surface addition of 1 in 10 diluted C-emulsion. A single above surface spare addition port directed to the vessel wall was also integrated for future flexibility. The splashing of culture on to the surface of the medium during inoculation of seed bioreactor should be avoided to prevent build up of foam. Therefore the inoculum addition port is above surface and directed to the vessel wall. The use of the harvest port in the base plate is the ideal port for removal of inoculum during transfer of inoculum. Additionally the medium addition port is directed on to the vessel wall. In summary the total addition ports are:

-   -   Four surface additions with medium inlet, inoculum inlet and a         spare small addition directed to the vessel wall and addition         port for antifoam dropped on to the liquid surface away from the         vessel wall.     -   Three subsurface additions for feeds and alkali.         Sample Ports

The sample port design is similar to that specified for the 20 000 litre bioreactor. The sample port is located 286 mm from tank bottom to minimise the volume that can be sampled.

Volume Measurement

The level sensor is able to measure up to 1000 litres. The level sensor sensitivity is at least 0.25% of full span.

Bioreactor Temperature Control

The 250 to 800 litres of medium is brought to operating temperature, typically 36.5° C. during initial inoculation and “seed rolling operation” by process control. This is achieved by “gentle” heating of the jacket and avoid high temperature at vessel wall.

Jacket

The bioreactor jacket area is specified with the following considerations in mind:

-   -   Steam sterilisation at 121-125° C.     -   Warming up of 250-800 litres of medium from 10° C. to 36.5° C.         in <2 hrs.     -   All points within the bioreactor must reach ±0.2° C. of set         point, typically 36.5° C. as measured by thermocouples.     -   Chilling of 400 litres of medium from 36±2° C. to 10° C. in 2         hrs.         Bioreactor pH Control

The process pH is monitored and controlled with probes connected via a transmitter to a DCS based process controller. The process pH is controlled by addition of CO₂ to bring the pH down to set point and addition of alkali to bring pH up to set point. Alkali is added through at least one subsurface port at centre-line of the bottom impeller. The CO₂ is added via the control sparger.

The control and back up probes are in the lower port ring at 286 mm from tank bottom to minimise the volume that can be monitored as shown in Table 24.

Bioreactor DOT Control

Dissolved oxygen is monitored and controlled with polarographic DOT probe. The DOT set point maintained by sparging: —

-   -   Initial N₂ ballast and/or air on demand     -   Air ballast with air on demand     -   Air ballast with oxygen on demand

DOT control allows DOT set point to be maintained through interchangeable use of oxygen or air as demand gas.

Control and back up probes are in the lower port ring at 286 mm from tank bottom minimise the volume that can be monitored, as shown in table 24.

Feed Addition Control

The point of addition is 286 mm from tank bottom, in the vicinity of the centre-line of the bottom impeller to assist in the rapid dissipation of feed bolus.

Antifoam Addition Control

The addition point is at surface projecting 0.25×T toward the tank centre or 216 mm from tank centre.

EXAMPLE 4: BIOREACTOR TRAIN

The bioreactor design is based on the ability to perform both 1 in 5 (20% v/v) and 1 in 9 (11% v/v) subculture ratios. The bioreactor train consists of a 1000 litre (Stages N-3 and N-2) and 4000 litre (Stage N-1) seed bioreactors followed by a 20 000 litre production bioreactor (Stage N). The operating volumes for each bioreactor are defined in examples 1 to 3. The bioreactors are based on a stirred tank design and a top driven agitator system was used.

The design is based on the need to ensure a homogenous environment with respect to process parameters such as pH, dissolved oxygen tension (DOT) and temperature, maintaining a well mixed cell suspension and blending nutrient feeds within the bioreactor. This provides the necessary physicochemical environment for optimal cell growth, product accumulation and product quality. Key to the design philosophy is the need to maintain geometric similarity. This allows a scale down model to be developed at 12 litre laboratory and 500 litre pilot scales. The design of the seed and production bioreactors are based on the same principles although some departures are required to allow for flexibility in processing. The aspect ratios (H_(L)/T) selected are typical of those used in mammalian cell culture and are in the range 0.17 to 1.96 post-inoculation.

TABLE 25 Key bioreactor design parameters 1000 liter 4000 liter 20000 liter Aspect ratio (H_(L)/T) 0.17-1.96 0.63-1.21 0.83-1.34 Impeller to tank 0.44-0.46 0.44-0.46 0.44-0.46 diameter (D/T) Operating Volume (L)  50-960 1914-3846 13913-21739 Agitator speed (rpm)  0-155  0-88  0-80 Control sparger CA (SLPM)  2-20  0-60   0-1000 Ballast sparger No ballast No ballast  0-500 CA/N₂ flow (SLPM) sparger sparger Cultivation  2-5¹ 2-5 10-15 residence time (days) Feed additions 2 surface 2 surface 4 surface 3 sub-surface 3 sub-surface 4 sub-surface ¹The culture residence time in 1000 liter bioreactor may be higher depending on the length of time the bioreactor is repeatedly sub-cultivated or “rolled”.

The design constraint is based upon a seeding ratio of 11% v/v (1 in dilution) and 20% v/v (1 in 5 dilution), with feed application of 4% v/v to 25% v/v of the post-inoculation volume. The post-inoculation volume in the production bioreactor is adjusted for feed applications up to 15% such that after the addition of all the feeds the final volume at harvest ends up at 20 000 l. However for feed applications greater then 15% v/v the post-inoculation volume is adjusted for a 15% v/v feed but following the application of feeds the final pre-harvest volume will be a minimum of 20 000 and a maximum 22 000 litres. The production bioreactor is expected to hold a total of 20 000 to 22 000 litres at the end of a batch. Table 26 shows the pre-inoculation volume, inoculation volume and transfer or harvest volume for each of the three inoculum expansion stages and the production bioreactor.

The seed bioreactors (stage N-1 to N-3) are unlikely to be fed therefore the maximum operating volume will be at inoculation. The operating volume range for the 4000 litre seed bioreactor (stage N-1) is to 3846 litres. In order to design a bioreactor that can grow cells from 20% v/v seed split ratio, the 1000 litre seed bioreactor (stages N-2 and N-3) will operate at two operating ranges. For the 11% v/v seed split ratio the bioreactor train can produce sufficient culture to meet forward processing cell concentration criteria in a single expansion/sub-cultivation stage. However the bioreactor train requires two expansion/sub-cultivation stages to meet forward processing criteria for 20% v/v seed split ratio process. Thus for 11% v/v seed split ratio process an operating range of 400 to 450 litres is required and for the 20% v/v seed split ratio process an operating volume range of 250 to 960 litres is required.

TABLE 26 Vessel sizes for bioreactor train 1000 4000 20000 liter liter liter Stage N-3 N-2 N-1 N 11% v/v Seed with 4 to 25% v/v production feed Pre-inoculation Volume (L) 400 — 1914 15456-17096 Inoculation Volume (L) 450 — 2153 17391-19231 Transfer or Harvest Volume (L) 450 — 2153 20000-21739 20% v/v Seed with 4 to 25% v/v production feed Pre-inoculation Volume (L) 250 768 2782-3077 13913-15385 Inoculation Volume (L) 300 960 3478-3846 17391-19231 Transfer or Harvest Volume (L) 300 960 3478-3846 20000-21739 Assumed operating volume Minimum Volume (L) 250 1914 13913 Maximum Volume (L) 960 3846 21739 Ratio of Maximum 3.84 2.01 1.56 volume/Minimum volume

It is recommended that the 1000 litre seed bioreactor is inoculated from culture produced in an S200 Wave bioreactor.

1000 l: This bioreactor is operated in batches of up to 5 days, with potential “shot additions” of feeds, for cultivation of mammalian cells. However due to repeated drain and refill operation at the end of each batch the total process residence time in this bioreactor can exceed 30 days. The mammalian cells are kept in a homogeneous suspension by agitation via an identical impeller system to the 20 litre bioreactor. Additionally other features will be kept geometrically similar to the 20 000 litre bioreactor, where possible.

Sparging air or oxygen and air or nitrogen respectively will control process DOT. Process pH is controlled by addition of alkali for base control and of sparged CO₂ for acid control.

The process operating volume of the bioreactor changes at different phases of operation. Initially the bioreactor is aseptically filled with a bolus of medium at 250 to 400 litres in 0.5 h. The bioreactor is operated in a pre-inoculation phase to bring the process variables to predefined set points. 50 litre culture from a (N-4) S-200 seed wave bioreactor is inoculated, by pneumatic assisted flow, or pumped with a peristaltic pump in 25 to 30 minutes into the 1000 litre bioreactor at 1 in 5 or 1 in 9 dilutions. The post-inoculation operating volume is 300 and 450 litres for 1 in 5 and 1 in 9 seeded process respectively. The addition of alkali for base control and 1 in 10 antifoam suspension for suppression of foam contributes towards the final volume. The inoculum culture may be fed by a “shot addition” if the culture interval is longer then expected. As a result of mixing and gassing the liquid volumes described above will expand due to gas hold up. The extent of this rise is dependent on the sparger type used, power per unit volume imparted by impellers and superficial gas velocity of sparged gasses.

The N-3 stage ends when viable cell concentration reaches transfer criteria. The N-2 stage for 1 in 5 process begins with a bulk up in volume to 960 litre by draining of 192 litre excess culture and addition of 768 litre fresh medium in 1.5 h. 696 to 769 litre of culture are transferred at the end of N-2 stage to the 4000 litre bioreactor for the in 5 processes. For the 1 in 9 processes 239 litres are transferred to the 4000 litre bioreactor.

The 1000 l bioreactor is continuously “drained and refilled with fresh medium” or “rolled” to provide back up culture for the 4000 litre bioreactor. The duration of the rolling seed operation is dependent on the length of the production campaign and the permissible elapsed generations number of the seed culture. Typically it is assumed that rolling seed operation is in excess of 30 days. The rolling operation consists of retaining approximately 192 litres of the 960 litre culture and diluting with 768 litre fresh medium for the 1 in 5 processes. For the 1 in 9 processes the 1000 litre bioreactor is expected to be “rolled” by retaining 50 to 100 litre of the 450 to 900 litre culture and diluting with 400 to 800 litre fresh medium. Process control ranges are relaxed over this operation. The medium added to the bioreactor during rolling operation is warmed to 30° C.

4000 l: This bioreactor is operated in batch of no more then 5 days, with potential “shot additions” of feeds, for cultivation of mammalian cells. The mammalian cells are kept in a homogeneous suspension by agitation via an identical impeller system described in example 1. Additionally this vessel is geometrically similar to the 20 000 litre bioreactor.

Sparging air or oxygen and air or nitrogen respectively controls process DOT. Process pH is controlled by addition of alkali for base control and of sparged CO₂ for acid control.

The process operating volume of the bioreactor changes at different phases of operation. Initially the bioreactor is aseptically filled with a bolus of protein free medium at 1914 to 3077 litres in 1.5 h. The bioreactor operates in a pre-inoculation phase to bring the process variables to predefined set points. Culture from the 1000 litre (N-2) seed seed bioreactor is inoculated by pneumatic flow at a flowrate to allow transfer in one hour, at 1 in 5 or 1 in 9 dilutions. The post-inoculation operating volume is 2153 to 3846 litres. The addition of alkali for base control and 1 in 10 antifoam suspension for suppression of foam contributes towards the final volume. The inoculum culture may be fed by a “shot addition” if the culture interval is longer then expected. As a result of mixing and gassing the liquid volume expands due to gas hold up. The extent of this rise is dependent on the sparger type used, power per unit volume imparted by impellers and superficial gas velocity of sparged gasses.

20 000 l: This bioreactor is operated in batch or fed batch mode for to 15 days for the cultivation of mammalian cells. The mammalian cells are kept in a homogeneous suspension by agitation via an impeller system.

The process operating volume of the bioreactor changes at different phases of operation. Initially the bioreactor is aseptically filled with cell culture medium at 13913 to 17096 litres in 1-2 h. The bioreactor is operated in a pre-inoculation phase to bring the process variables to predefined set points. Culture from the 4000 litre seed bioreactor (N-1) is inoculated by pneumatic flow at a flow rate range of <4000 l/h into the 20 000 litre bioreactor at 1 in 5 or 1 in 9 dilutions. The post-inoculation volume continuously increases following an application of sub-surface feeds to maximum of 20 000 litres (two feeds totalling 4 to 25% v/v). The addition of alkali for base control and 1 in 10 antifoam suspension for suppression of foam accounts for about 100 litres and 20 litres respectively. As a result of mixing and gassing the liquid volume expands due to gas hold up. The extent of this rise is depended on the sparger type used (fluted or sintered), power per unit volume imparted by impellers and superficial gas velocity of sparged gasses.

Table 27 describes the aspect ratios in the 20 000 litre bioreactor at various operating volumes during normal processing. The aspect ratios have been tested at 500 litre scale and provided the superficial gas velocity and power per unit volume are kept constant the K_(L)a remains constant.

TABLE 27 Key operating volumes and aspect ratios in the 20 000 liter bioreactor Volume, Liquid head, Aspect ratio, L mm H_(L)/T Pre-Inoculation 13913-17096 2458-2977 0.88-1.07 Post Inoculation 17391-19231 3025-3325 1.08-1.19 Harvest 20000-21739 3451-3734 1.23-1.34 

What is claimed is:
 1. A scalable bioreactor for the cultivation of mammalian cells, said bioreactor comprising: a biocompatible tank or vessel having at least one top impeller and at least one bottom impeller, wherein the top impeller is a hydrofoil impeller, wherein an impeller spacing (D_(s)) between the at least one top impeller and the at least one bottom impeller is between 1.229×the diameter of the bottom impeller (D_(bottom)) and 2×D_(bottom), wherein the height of a liquid above the at least one top impeller (D_(o)) is between 0.3×the diameter of the at least one top impeller (D_(top)) and 2.5×D_(top), and wherein a bottom clearance (D_(c)) between the tank bottom and the center-line of the bottom impeller is at least 0.35×D_(bottom), wherein the scalable bioreactor allows for performance and product quality independent of the size of the bioreactor.
 2. The bioreactor according to claim 1, wherein the top impeller is a three bladed hydrofoil design impeller, and the bottom impeller is a four pitched-bladed high solidity impeller.
 3. The bioreactor according to claim 1, wherein the impeller to tank diameter ratio is at least 0.35 and at most 0.55.
 4. The bioreactor according to claim 3, wherein the impeller to tank diameter ratio is at least 0.40 and at most 0.48.
 5. The bioreactor according to claim 3, wherein the impeller to tank diameter ratio is at least 0.44 and at most 0.46.
 6. The bioreactor according to claim 1, wherein the bioreactor has at least one sparger.
 7. The bioreactor according to claim 1, wherein the bioreactor has at least one baffle.
 8. The bioreactor according to claim 1, wherein the bioreactor has at least two ports for alkali addition.
 9. The bioreactor according to claim 8, wherein said at least two ports are spatially separated from one another.
 10. The bioreactor according to claim 1, wherein the bioreactor maintains a homogeneous environment such that: a) pH is maintained within +/−0.03 of a pH set point; b) the dissolved oxygen tension (DOT) is maintained with +/−2% of a DOT set point; and c) temperature is maintained within +/−0.2° C. of a temperature set point.
 11. The bioreactor according to claim 10, wherein the temperature set point is from 36 to 38° C. 